One sequence lagging...

We have one sequence that lags. It currently runs about 15 minutes, multiple tracks, proxies, low res playback, all settings optimized, 3rd party effects rendered.  We get the spinning ball for a second or two then playback.  Sometimes cursor won't let go of posion bar until a couple of clicks. All other sequences with same footage and simar effects play fine.  Multicam has given us some trouble in the past.

We are using multiple projects in a production. We reimported the project into a new project and have copied the sequence into a fresh timeline. We have deleted all render files and re-rendered.

Mac Studio M1 Ultra, OS 14.5, 128GB RAM, plenty of internal and external SSD storage, Premiere 24.5.

Any wisdom?