Hello,
I think I might have found a bug while playing around with Drizzle/Drought and Swift Swim.
I find it hard to explain so I'll go straight into the details of what happened.

Swift Swim Kabutops versus a Mega Charizard-Y.

Turn 1:
HA Politoed (Drizzle) vs a Charizard which is holding Charizardite Y.
Politoed is sent it, rain goes up.
Politoed gets switched out, Kabutops comes in while Charizard stays in its regular form, without mega evolving, and goes for a Roost.
Rain is still up, of course.

Turn 2:
Charizard mega evolves into Charizard-Y, gaining Drought as its ability, and attacks Kabutops.
Charizard is faster than Kabutops and its attack goes first.

This shouldn't happen according to this replay:
http://replay.pokemonshowdown.com/customgame-563795728

As you see, Kabutops still outspeeds Charizard-Y even in the Sun, as the speed is calculated as the turn starts.

In Pixelmon, Charizard-Y is faster than Kabutops, I assume because the recognizes that the rain is gone and acts like Kabutops isn't affected by its ability Swift Swim, making it slower than the opposing Charizard-Y.

I hope I explained the situation in a clear enough way.
If more informations are needed, don't hesitate to contact me.

~ Xasah