Foam tyres made my car slower?

I took on some advice from the lovely people in this subreddit and got some 1/10 foam wheels for my speed running

I wouldn’t guess the wheels themselves would be the issue? Not entirely sure.

I got 85kph with foam tyres they were alot more stable then rubber wheels.

I used wltoys 1/12 wheels for speed running also and I got 105kph with rubber wheels, aren’t foam meant to be better and increase speed and grip? Btw they’re the same diameter