Extremely poor performance in Premiere Pro 2023.6.4

A post-production editing mantra I've held dear for many years is to NEVER update the software in the middle of a project

Too true.  The only time we suggest it in mission-critical environments is if we know changes have been implemented in newer versions that will directly address an issue head on, especially if that issue is affecting workflow. Of course we always strive for this transition to happen without risking significant fallout, but you're right... normally, just as a measure of safety, productions just stick on a version and upgrade for the next project.

No two projects are the same, and I can't claim to know the details of specifically what Premiere Pro features you are using versus how others may work; that said, we have moved other projects from v23 to v24 with success.  As @Kevin-Monahan mentioned, you can run v23 and v24 side by side.  You can't downgrade a project from v24 back down to v23, but if you have the freedom/time/interest, you can experiment with copy of your project in v24, while continuing your day-to-day work in your v23 project, to confirm that both the experience will be imporve in v24 and the migration will be successful. Assuming this testing confirms that v24 is a stable and benificial move, you can upgrade when you're ready to do so.

Separately, here are instructions for pulling  process samples on a Mac.  You can email them to me once you've pulled them.  If you can also detail what you were experiencing at the time, or what steps created the slowdown, that's also helpful:

1. Open the Activity Monitor application.
2. Find Premiere Pro in the process list and select it.
3. Click on the the ••• menu at the top of the Activity Monitor window.
4. Select Sample Process from the menu that pops up.
5. A window containing the pulled Sample will pop up, which will have a Save... button in the top right.

You can take as many samples as you'd like by repeating steps 4-5.  In fact, it's actually better for us if you pull a few so that we can confirm consistency across them.  If you press Sample Process (step 4) multple times, each resulting Sample window will cover up the last making it appear that the new sample replaced the last, but they are each unique samples.  You can then cull through the open unsaved sample windows and save each one as its own file.  If you remember, save them with some identifying name of what was happening for reference, instead of Sample 1, Sample 2, Sample 3.