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If you want to make Slow Start a real ability (since Regigigas is already a legendary idk why you would make it a detrimental one):
Just makes it that he starts normal so after the 5 turns it's just a plain x1.5 to Speed and Attack. It's not nearly as broken as one might think since 5 turns is a long fucking time so the situation is going to be rare anyways.
If you insist in making Slow Start a negative ability:
Either
-He gradually recovers his stats back (so after one turn he is at x0.6, after two turns at x0.7, etc)
-Make it so that the numbers of turns don't get reset after he switches out
