After ruku, by the time the imam finishes tasmi, he moves quickly to sujud, so I’m still rising when he starts (without takbir yet). Does this delay invalidate my rakah? If yes, what about past prayers, and can I rise with him to avoid it? I try to move after him but he’s very quick.

Ustadh AbdulAziz al-Haqqan

Just follow him u know he is fast so be quick to follow him.