Theres a reason Regigigas has Slow Start.
Because he was frozen in a temple for thousands of years. It's a bit tired and needs a while to get rejuvenated. If your in the hospital for being nearly killed... Is being one of the most powerfull creatures alive and instantly crushing the first thing you see in front of you the first thing you do after you wake up? No? Thank you.
Lets listen to the story....
Who read the Braille in the Sealed Chamber in R/S/E?
After you use dig, there are a bunch of stones. They read this...
In this cave we have lived.
We owe all to the Pokémon.
But, we sealed the Pokémon away.
We feared it.
Those with courage, those with hope.
Open the door. An eternal Pokémon awakes.
First comes Relicanth.
Last comes Wailord.
Who is this Pokemon? None other than Regigigas.
The three Regis sealed Regigigas away. Think for a minute... If it didn't have Slow Start, Regigigas would be nearly the Best Pokemon in the game. It is great Defensively, and if it didn't have Slow Start... Well, it would be AMAZING offensively as well. Anyway... Regigigas is the king of the Regis. It created them. They owe everything to the Pokemon. But, with it's amazing power... They feared it. So they sealed it away. The Regis made themselves the keys to unlocking it. To make it even harder to get... They decided to seal it away in a whole different region FAR away. (Although since this is a video game... Pal Park makes it easy to get them to Sinnoh. If you were to imagine this was real life though... yeah, it would be hard.) After it was sealed, they sealed themselves away too, to make it as hard as possible to find Regigigas. They said though, if anyone had courage and hope... Challenge the beast. How? Well, it takes a lot to capture it. But first comes Relicanth... Last comes Wailord...