You're having a laugh! To get from the concourse at Kings + to the Victoria line is at least a 5 minute walk for me, and I always walk down the escalator and weave in and out of people, or walk on the walls to avoid people walking slowly.
I'd get the Northern line from Waterloo to Euston; it's a direct train and the Charing + branch of the Northern line is very rarely busy at the levels of the Victoria line. I get a seat every morning. Admittedly the Charing + branch is quite slow because of the number of stations, but in the morning you very often have to let a Victoria line train leave because it's so busy (although that's going Southbound not Northbound).
I'm not having a laugh - are you sure you understood me?
For Paddington the Circle and Hammerline trains are infrequent and slow. Usually getting checked at either Baker St and/or Edgeware Rd. Jump onto the bakerloo line and the trains are fast and frequent. Same for the Vicky line. Not just me - almost everyone at work agrees these are the best routes.
Quickest way from Waterloo to Kings Cross is:
Bakerloo - Oxford Circus - Victoria Line - Kings Cross. By a LONG way it's the fastest. Back of Bakeloo line train, at Oxford Circus turn left, and as soon as you're roud that corner you'll see the back end of a Vicky line train. Get on there, and that's you at the closest exit for Kings Cross. I've been doing it since 2003, and trust me, it's the fastest way from Waterloo to KGX.
It even beats Northern Line to Euston 9 times out of 10 if you get of the Victoria line that one stop earlier. People assume direct routes are the quickest on LUL - sometimes yes, but not always and not in this case.
But unless you're elderly or disabled, getting from the concourse at KGX to the Vicky line platform takes ~60 seconds. Exit front of station, turn right down stairs, at bottom of stairs make a hairpin right and down the next stairs. Through the ticket barrier, take the right-hand escalators (walking down them of course) and then on the left is the Victoria line platform Southbound.
I'm not arguing about seats - I rarely if ever get a seat, but for a few minutes in rush hour - who cares?