[Cat Heller] “trying to make a video where 144 small clips play at once…mostly shot on smartphones…resize them to 11% and desaturate them..”
The below article discussed doing this for 64 concurrent videos on a top spec 2013 Mac Pro. You are doing over twice that.
“Building a 64 layer chessboard project in Final Cut Pro X”: https://www.fcp.co/final-cut-pro/articles/1472-building-a-64-layer-chessboard-project-in-final-cut-pro-x
If I understand correctly, you are playing 144 concurrent video streams simultaneously. Is that right? If so that is extremely challenging — on any NLE or any hardware. This would especially be so for H264 which is likely what the smart phones use. Some may even be HEVC which is much more compute-intensive than H264. If any of those are 4k it’s even harder.
It might be best to transcode everything to 1080p ProRes or create proxies for everything (proxies are 1/2 the linear and 1/4 the area resolution), so proxy of 4k is 1080p and proxy of 1080p is 960×540. This is usually adequate resolution for editing and will greatly increase performance.
If you are already using proxies, the only solution might be get a faster machine. You will also likely need a high bandwidth I/O system. Even if all your content is 1080p/30 H264 at the iPhone spec of 130 MB/min, this still equates to 2.2 MB/sec * 144 or 317 MB/sec, with nothing left over for anything else such as cache, library I/O, etc.
Or — if your video tiles are always small and never grow to a larger size, you could possibly use Compressor or Handbrake to transcode those before ingest to a lower resolution, such as 480×270 or smaller. That would decrease the CPU and I/O burden.