There was a bug in the handicap feature released in the last update which caused undesired input lag for handicapped players. This bug doesn’t happen if you are the host and that was the only way in which I had tested the feature before release, sorry!
I just released an update which fixes the bug.
So if you had tried the feature and thought input delay was normal you now know it is not and should give it another shot.
(You might need to clear your browser cache to ensure that HaxBall is now using the latest version.)