Toxic Spikes requires two turns to be effective, like you said, but it still has two immunities.
So basically two Pokemon types are not particularly ruined by it, but everything else is. You don't see any issues here?
I think yo're overestimating how crucial speed is. Sure, it is the most important stat, but it's not as important as you might think.
I think you're underestimating it. The fact that you think Charizard isn't particularly slow is telling.
PRediction isn't really a good counter. The Rapid Spin argument is that it's a waste of a move. It's a weak move that's practically required if you carry a Fire, Bug, Ice or Flying type. It's very specialized and doesn't even hit Ghosts.
Prediction isn't about countering. It's about prevention.
The Rapid Spin argument is bad. You don't refuse to use a move that has a useful effect just because it deals little to no damage. Whirlwind and Roar are prime examples. it doesn't hit Ghosts, but that's one type out of 18.
As for YZard, how is getting rid of 50% of a pokemon's health "balancing"?
As established, it hits like a burning truck and has one of the most powerful abilities in the game.
"Murder of the living is tragic, but murder of the idea is unforgivable." - Janus, speaker of the synod