I found the problem and unfortunately it's not a problem that i can fix (or at least, not so easly fixable...), it's a problem shared whitin all unity WebGL games really...
I could speed up the player speed and slow up the slimes, but then other player may experience the polar opposite problem... (performance based problem)
The problem is influenced by many factors, so try changing browser, or pc if you have the possibility...
Again, sorry for the inconvenience... =C