I would expect everyone going at different speeds in different lanes to increase congestion, because it encourages people who want to go faster than the general

I would expect everyone going at different speeds in different lanes to increase congestion, because it encourages people who want to go faster than the general flow of traffic to jockey around a bunch, which then adversely impacts the speed at which everyone else can go as they react to all those sudden moves, which ultimately slows everything down even further. Not entirely unlike how turbulence in a pipe decreases the overall rate at which water can flow through it.