Playback lag in record monitor only

Hello everyone,

Something is happening where when I press the space bar to play a sequence the playback lags. I also get delays and spinning color wheels when I sometimes make edits or  try to move sequences around in bins. Playback in the source monitor is perfect; video plays immediately upon pressing the space bar.

I think it's something related to my sequence settings because the lag happens only when I play a sequence, but I can't figure it out.I think I have the sequence settings match the source media settings, but I can't help the feeling I'm missing something there. 

- The lag depends on the number of clips in the sequence. Only a few clips results in a small lag. But a 10 min sequence with many cuts results in a 4-5 second lag. 

- I have 1/4 res playback.

- I'm using some proxies. In the process of creating proxies for all clips.

- My drive doesn't seem to be the problem as source playback -- even in 5 camera multi-cam -- is smooth. (Tested it and got 220 MB/sec.)

- Source clips are 1080p, AVCI 100.

- I've tested sequences set to AVCI 100 and ProRes 422. Both show the problem.

- I've reset prefs, deleted the cache, repaired folder permissions, rebooted the Mac, copied the project to a new one.

My system: 

27" iMac

72 GB RAM

12 TB USB superspeed 10 drive. 

Premiere 23.6.2 and 24.0 (Have tried both versions)

Thanks!