Timeline Playback stuttering/skipping only when rendered?

Turn off background rendering and leave it disabled. It's not necessary, especially on a fast machine like you have. If at some point you have trouble playing back a section of the timeline that's very heavy with effects, manually render just that section by selecting it and typing ⌃R.

You can recover a lot of disk space used by all the render files by selecting the library in the browser and choosing File->Delete Generated Library Files... and selecting everything. Manage render files in Final Cut Pro for Mac - Apple Support 

As Luis suggested, it might be a good troubleshooting step to remove Chrome and the related daemon processes. Those have been previously associated with misuse of the MacOS Video Toolbox framework, which can block other video-oriented apps from running smoothly.

It appears you have three monitors on that system. That should be OK, however a past FCP problem happened with multiple monitors. I thought that was fixed, but if you could temporarily remove the two other monitors and leave connected the Apple Studio monitor, reboot, and try FCP, that would help rule that out.

If it still happens after the above steps, I'd like to see about 15 seconds of the system log during the problem. It will be very verbose, but I can use BBEdit to progressively filter through that. Procedure:

• Get FCP ready to reproduce the problem.
• Get this terminal command ready to run: sudo log show --last 15s > ~/Documents/LogShow15s.txt
• Reproduce the problem in FCP
• Once it happens, immediately run the above 'log show' command.
• The problem need not happen while running the command,since it looks back in time for 15 seconds, but it must have happened within the previous 15 seconds.
• Zip that file, upload it to wetransfer.com and post a link here.

If this doesn't show anything, there are other steps to take, but these are the most straightforward initial steps.

Very strange.

A common trouble-shooting technique when FCP behaves strangely is to reset its settings. Hold option and command when launch and choose Delete Preferences... It will open the default Untitled library. Reopen the library you were working on and set the preferences back to the way you like. Be sure to Disable Background Rendering again.

Another technique is to run an https://etrecheck.com report and post it here by clicking the Additional text button and pasting it. This anonymized report helps the hivemind here to diagnose common problems.

I have an M1 Ultra, and I've never seen this. When the timeline is rendered to the cache, it should be smoother.

Is it just laggy or does it also hang with a "beachball" cursor for periods? If so, for how many seconds?

Please list the specifics of the external disk holding the libraries. If the internal SSD holding the media is fast, and the external disk holding the libraries is slower, this could happen: Before rendering to cache, it will be reading off the fast internal SSD. After rendering to cache, FCP will be reading from the external disk, since by default cache files are stored in the library.

Another effect is cache files are optimized ProRes 422. That means if your original media is 4k H.264 or 4k HEVC, after rendering, those cache files on the external drive may be about 6x to 15x the size of the original files. That means not only might those cache files be on a slower drive, the required bandwidth to read them could have increased by 6x to 15x.

I just did a quick test with a 4k/23.98 HEVC file with a reading I/O rate of 5 megabytes/sec at 1x playback. When I rendered that to cache, it produced 16GB of cache files, requiring about 10x the I/O rate of the original HEVC file. That was still "only" 50 megabytes/sec, so most 7200 rpm mechanical drives and all SSD drives should be able to handle that. However if you're dealing with multicam, it would be multiples of that.

Thus, we need to know the specifics of that external drive: SSD or mechanical, interface, etc. If it is USB is it directly connected or through a hub? Can you run the free Blackmagic Disk Speed Test on that drive? (Not the RAW speed test, the regular one).

You have several slow drives connected. If these drives take time to spin up, even if FCP is not using them, they could slow down things.

Try a library in the internal drive, and with the external drives disconnected. Same problem?

One more item: we need to know the Fx in use at the timeline region where it's slow. Select that timeline clip and do Edit>Remove Attributes, but DO NOT remove them, just make a list of them. Scroll down the dialog if needed to see them all. Post that list here.

If it is a connected clip, compound or multicam clip, please open the clip and do the above command on each of the internal clips, then post here the Fx on each of those.

Please also state whether any clips in that timeline region use optical flow or the object tracker.

Normally a clip with no Fx cannot be rendered. IOW when you import it, no render dots will show in the timeline. Usually you must add some effect or transform. Did you do that? If so which one?

You can select the timeline clip and do CTRL+R to render that, assuming it is showing render dots above the clip. Otherwise it won't do anything.

If you can reproduce this with one clip, create a new project, add the one iPhone clip, and verify it exhibits laggy behavior if rendered. If so export a project XML for that little one-clip project and post that as an attachment.

If it does not happen with a one-clip project, add more clips or otherwise examine the difference in project characteristics between the test project and the original project.

I've never seen that before. After rendering has completed, playback should be smooth. It's possible the render files have filled up your disk. How much free space do you have?

How is the library HD formatted?