Lag when copying or adding sample/midi in Logic pro x

I have a recurring issue since 4 days.

In my current logic pro x project everything runs smoothly (for instance playback, bouncing, starting, moving midi or sample left or right) but whenever I add a midi/sample or copy/paste a midi/sample logic is not responding and the rainbow wheel occurs for 1-2 minutes. After that everything is running again. Also the saving of the file takes that long with a rainbow wheel. Furthermore in the activity log logic runs wild and takes 150% CPU, 25GB Ram - but only with this project. everything else is working fine.

What I have tried so far:

• I updated Logic to its current version
• I changed Buffer size, audio engine, multithreading
• I set back to a backup that was 7 days ago. Started to work with the same file again - accepting that I have lost 7 days of work and the error came back about 2-3 hours later
• I deleted the undo history, deleted unused files and exported it to my external drive.

I have worked on this mac with bigger projects (130-140 tracks and 700+ plugins) - everything has run smoothly so far. I don't get it.

Fingers crossed that anyone can help me getting back to my workflow.

Thank you in advance!