In my parents time after electrification the two tracks from Streatham to Sutton had 7/8 peak trains per hour, express and all stations. Off peak it was 4/5 per hour.
The shoulder peak hour trains took c1hr 34mins to get to Bognor in SR days although the fastest peak took just 1hr 27mins including dividing at Barnham from the PH portion.
Moving forward to the early 80s, the 1720 took 1hr 40min stopping at Sutton Dorking Horsham and all stations to Bognor except Christs Hospital.
It is reported on Wikipedia at
http://en.wikipedia.org/wiki/Dorking_railway_station that the fastest ever service from Dorking to London Victoria was 34 minutes immediately following electrification of the entire Mid Sussex line in 1938. Things then probably did not change very much (although I expect that in the intervening period virtually all timetabling and line infrastructure improvements were focused only on services from London Victoria to Brighton via Redhill and Gatwick) until May 1978 when mid-Sussex electric services were re-routed via Gatwick Airport.
Certainly when I first became aware of services on the line when I considered moving to a part of Capel not far from Ockley station in 1991 (at which point the booking office at Ockley had already been closed for some years but not so far converted in to an office or house) the fastest off peak services from Ockley to London Victoria or vica versa took 51 minutes and the very slowest peak services took 60 minutes exactly. As the Ockley to Dorking part of the journey was historically always timetabled at 10 minutes this implies there was actually only a slow deterioration over 40 years in journey times from Dorking to London Victoria from a fastest possible journey of 34 minutes in 1938 to 41 minutes some 53 years later in 1991.
Even after initial privatisation and operation by Connex in the mid 1990s relatively little happened to worsen or further slow down journey times. It was only once the GOVIA group got its hands on services between Dorking and London Victoria twice an hour and Horsham and London Victoria hourly that journey times began to slow down and slide backwards hugely from one timetable to the next, largely because GOVIA was financially incentivised to go on increasing journey times to a point where all financial penalties for late running and/or cancellations were minimised. And whatever GOVIA wanted in terms of worsened minimum operating specification for the line they always seemed to get from the powers that be.
This means that over the 23 years since 1991 the fastest possible journey time from Dorking to London Victoria has increased from 41 minutes to 50 minutes but typical journey times for most passengers have slid even more than this as the fastest possible rush hour journey from Dorking to London is now 56 minutes to travel only 22 miles!
All of this seems to have come about because GOVIA/Southern's main focus on the lines out of London Victoria via Sutton seems to extend only as far as Sutton and possibly Epsom where they clearly aim to run the effective railway line equivalent of an underground train service with trains nearly full up with passengers stopping at all stations on the line to adequately handle capacity demands during the morning peak. Capacity only focused developments such as the creation of Mitcham Eastfields have also now managed to add another couple of minutes to many of the journeys on the line.
As far as I can see Southern's senior management (who I have taken the time and trouble to meet on several occasions up at their Meet The Manager sessions at London Victoria) are simply not at all interested in the needs of passengers travelling on the Mole Valley line corridor from Leatherhead and Dorking and also South of Dorking in to London Victoria and instead regard them as an inconvenient after thought to their main objective of running an Underground type stopping at all stations service out of Victoria via Sutton where their only main concern is train capacity and train frequency on the route with no thought at all to total journey times or any frequently repeated increases in those journey times over the years.
Even the rolling stock strategy is utterly confused with Southern services to Dorking and Horsham still served by a combination of the new Electrostars and the South London Metro type stock without any loos (whatever the train type numbers for those are) and some former Thameslink type rolling stock (the stuff with the very low ceilings near some of the ends of the carriages).
In my personal opinion if Southern's management were actually at all interested in or cared about the needs of their longer distance travellers from Leatherhead and Dorking and south of Dorking then it would be perfectly possible to create a train path for a once an hour Mole Valley line special probably stopping at all stations from Horsham through to Leatherhead but after that stopping only at Clapham Junction. However to do this might require both reintroducing the passing loops at Cheam (surely an utterly trivial capital cost in terms of total Network Rail investment and probably costing significantly less than the recent overbridge and lifts at Dorking Station) and also probably substantial efforts at re-timetabling the line from Sutton to Clapham Junction to clear the necessary hourly higher speed through path for this service.
When some of you talk about the issue of the embankments and maximum speeds that are possible on them by trains south of Dorking you surely have to be joking since as one used to the horrid slow crawl of a service now taking up to 79 minutes from Ockley to London Victoria (slowest journey time only 20 years ago 60 minutes) the only place on the line where the trains ever seem to travel over 50mph is actually south of Dorking or occasionally between Dorking and Leatherhead. Even then the trains only travel at any speed when they are running late after being held up in the London suburbs as Southern's timetabling is so ludicrously over cautious for the whole line that trains frequently arrive at stations on the southern parts of the line three or four minutes early and then have to wait for the timetable to catch up with them if they ever try to run at anything approaching maximum permitted speed (which some drivers still do just for the hell of it even though the slow as a snail timetable usually makes it pointless).
So in short services are in my opinion slow and snail like between Epsom and Horsham not due to any real physical constraints but simply because this section of line is completely forgotten about by Southern's management, who seem to be focused only on those stations and line sections in their network that handle the very largest volumes of passengers
EDIT:- It is interesting to note that it in fact always seems to have been possible for a train to cover the 36 or so railway miles from London to Horsham via Sutton and Dorking pretty quickly long ago if it made few or no stops and a path along the line was cleared in the timetable so that the train could run throughout at or close to its maximum speed.
For instance the 1905 LBSCR timetable available at
www.lbscr.org.uk/timetables/index.htm shows that a Mondays only train at 10.57am from Horsham took only 53 minutes to cover the 36 or so railway miles involved travelling non stop to London Victoria. It is hard to speculate on the exact purpose of this non stop train from Horsham although given that Parliament in those days did not convene till the afternoon and then sat late in to the evening it is relatively easy to have visions of some kind of MP's special, depending on who exactly had influence on the Board of the railway company in those days. It is also interesting to note that some 109 years ago when simple early steam trains were still in use, the fastest possible journey time for a normal passenger service from my own local station of Ockley (29 railway miles from London Victoria) at 7.45pm on a Sunday evening was just 59 minutes, four minutes faster than any train today is now able to make the journey today with Southern's Electrostars!
In fact by the 1917 LSBCR timetable this journey time on Sunday evening at 7.45pm had now improved by one minute to 58 minutes. However by 1917 the MP's special from Horsham on Monday morning only at 10.57am had gone but been replaced by a now daily 10.59am train from Horsham to London Victoria making only 2 stops (at Sutton and Mitcham Junction) taking only 55 minutes compared to the previous non stop 53 minutes.
It is especially worth noting that in those days the journey from Sutton to London Victoria only took 16 minutes, even whilst stopping at Sutton and Mitcham along the way. As this train ran so much faster than all of the others it would be interesting to find out precisely what kind of Steam Express monster of the time it was pulled by. Regarding the very fast journey times (for those days) from Ockley on a Sunday evening (when far fewer trains would have been running given the then almost wholly religious observance focused nature of this day) this is probably also a comment on how much difference to journey times can be made by a timetable slot deliberately cleared to allow maximum speed running by a train. Of course 29 miles in 58 minutes with only two stops ought to have been a perfectly viable proposition at top speed, even with the much slower steam engines of the time that were then in use.
... if she was not kidnapped for test trains 
