Lumetri Color works very slowly

Can you provide more details about your media? Specifically, the codec and frame size you're working with?

I tested Lumetri Color on a multi-camera sequence using ProRes HD footage on a relatively old and underpowered MacBook Pro. Despite the hardware limitations, adjustments in the Lumetri panel were smooth and responsive.

Knowing your media specs will help identify if the issue is performance-related or possibly linked to another factor.

Lumetri is a very heavy effect in computational load. Not quite as heavy as Warp, which even exceed Neat video Noise Remover ... but getting close.

Multicam is throwing however many different vide clips at a time at the computer's capabilities, which can stress many rigs. Now throw in a massive effect like Lumetri ... you see the problem?

But past that, I never do color work on the multicam 'sequence' itself, it's a time suck.

I drop the used clips on a new sequence, trim to just a couple seconds of each ... grade each clip to neutral then to match ... copy each clip's Lumetri, and paste it on that clip in the bin. Delete that shor little grading sequence.

This is a "source effect", which now is applied on all instances of that clip anywhere in that project.

And done that way, doesn't seem to slow up my rig while cutting the multicam.