So I downloaded 10.4 yesterday and yesterday evening I recorded my daughter's concert performance. I loaded that into 10.4 last night and cut a new video of about an hour in length. Nothing dramatic, single 1080p video at 72mbps with overlaid audio and two layers of titling and graphics.
I was finding it fairly laggy and it even dropped frames. This is on a 2017 iMac 3.4 i5 with the 2TB fusion and 40GB ram. The previous 10.3 version of FCPX was buttery smooth. 10.4 was not.
Now one could be a conspiracy theorist and suggest that Apple is making it worse so encourage users to get the iMac Pro. Coincidence?