The Kingston loop from Wimbledon to Richmond, another route I've had to do by bus, is considerably less complicated, and is not greatly slower than the train. However, the obvious route (on a map) from Norbiton to Kingston goes under a low bridge, and conspicuous signs are put up at Norbiton warning drivers of this. Nonetheless, some time back one driver didn't take note, and his bus managed to hit the bridge.
I usually drive the New Malden to Twickenham rail replacements when the line is down. Norbiton is my least favourite part of the route as the forecourt can be a bit of a farce with people parking outside the Sainsbury's local. It also involves reversals and three point turns, as well as the negotiation of narrow roads. Some drivers reportedly skip Teddington and Strawberry Hill because it's too difficult for them :roll:. There was an incident with a driver crashing a bus into that bridge at Norbiton on the first weekend in January this year, reportedly a contributory factor to that operator's removal from the contract to operate that route for the next few weekends.
I also drive Clapham Junction to Surbiton (via Wimbledon), Clapham Junction to Twickenham (via Richmond) and occasionally Barnes to Feltham via Hounslow. The running times are the same throughout the day - barely enough with the daytime traffic, but lots of slack in the evenings. I tend to hang on at the bus stops until 30 seconds after the scheduled time. The fun thing about these routes is that improvisation is often required due to roadworks or plant machines blocking level crossings at Mortlake or North Sheen.
I find that I'm always overtaking other buses on high frequency rail replacements. I overtook around 5 buses in one trip when I did Tower Hill to Barking the other week. Some drivers don't know where they're going. When I catch up with a pair of buses, it's because one is following the other, which slows both of them down. I overtake them when I see this happening. The official route between Stratford and Upton Park (via Plashet Grove) had abysmal traffic management which impacted on the service. It's a mystery why Romford Road and Green Street aren't used instead.
Sometimes the time difference can be horrendous. I remember when I drove Hammersmith to Northfields. TfL brand it as a Picadilly Line replacement, although it's actually all District Line stops to Ealing Broadway then straight to Northfields. Hammersmith to Ravenscourt Park takes 1 minute on the Underground, but took 40 minutes on that day due to roadworks

hock:. I don't know how people cope with driving in inner London, all of the weekend LU replacements I do seem to involve sitting in gridlock for substantial periods of time! I think I preferred driving routes like Finchley Road to Canons Park (via the Northern Line Golders Green to Edgware) and Wembley Park to Northwood. Wimbeldon to Hammersmith isn't too bad either.