Are we allowed to use the "fast craft" when doing a Minecraft Speedrun?

ur allowed to use the in game recipe book yes

Yes you can. It’s not very fast though, I’d recommend you learn to craft using hotkeys and muscle memorise important crafting recipes you need in a speedrun like tools, bucket, barrels, boats, doors, etc.

Recipe book is good for crafting items individually but if your gonna craft more items it can be faster to do it manually, also you can open the recipe book by going in your inventory then press tab and then spacebar

as a professional STK hater, I must say the fastest method for crafting is manually