Yep this is a "problem" with buses in London. They stop all the time so you never get beyond about 15mph before you start slowing down again for the next stop. The 100% EV buses are better as they accelerate faster, but not by much.

They are starting to introduce "superloop" buses that interlink the more suburban "spokes" for tube & train so you can go "across" London without having to go to the centre and then back out again. I.e. basically express buses that only stop at tube & train stations and other major interchanges.

It is a great idea, but it has been done for political and ideological reasons. They have not introduced express buses for people who work in central London for example (so instead to get to central London on a bus from a suburb means stopping every hundred meters or so, so even with bus lanes progress is brutally slow). These buses going "across" are not actually that useful for most people I expect because people using transport for high-paying central London jobs will be going "in" not "across". It is the "working class" lower paying jobs that might benefit from sideways interconnects between the suburbs - this is the ideological/vote-buying reason they've introduced them.