I can no longer play my timeline smoothly after I did a color correction. I never had this problem? They are files from my DJI Mavic 3 and Sony A7Siii (recording on a Atomos Ninja V), all 4K 50P. When I put the files on my timeline, playback is smooth (I also add a camera lut). I do a color correction, playback gets very choppy. Files are already optimized! Playback is already set to 'Better Performance'. A few weeks ago I didn't have this problem? Who has a golden tip?
...HHD: video footage: Samsung T5 2TB, connected through an USB hub, (multiple HHD drives, mostly Samsung T5)
Ideally connect each T5 drive directly to the iMac using the original short, thick OEM cable, not a hub. The T5 comes with both USB-C to C and USB-A to C cables. There have been many performance issues on the T5 and similar drives caused by using non-OEM cables or hubs.
An alternative approach is run Blackmagic Disk Speed Test on each drive. Report the read/write performance of each one.
This is not guaranteed to fix the problem, but it's a standard troubleshooting approach in this situation.
Another possible cause is the codecs of the Mavic 3 and A7SIII can be very compute intensive (depending which ones are used). You said the A7SIII records on the Ninja V -- does that mean ProRes 422, or what? The Ninja V can record many different codecs, including HEVC. If it is recording 4k ProRes 422 HQ, that has a high data rate, and if combined with a hard drive bottleneck due to an SSD cable or hub, that could slow things down.
Also make sure that each drive (inc'l system drive) has at least 20% free space.