Running FCPX 10.4.6. on a 2013 iMac with High Sierra, normally a no problem setup. Simple project with 2 XDCAM -EX files playing at the same time: same camera, same codec, same framerate (720P50), no effects.
From a certain time on, we experience choppy playback, lagging, hiccups, where on othe times everything plays fine, even with more tracks. Did everything you can think of (updated plugins, removed prefs and presets, new timeline, new user) all at no avail.
Made the upper set of clips into a compound clip: BAM!, smooth playback! Open the compound clip in the editor: choppy... What the heck? Is it the fact that the compound clip sets new temporary properties to the nested clips? That would mean there's something fishy with the original files, but they have all the right properties, play smooth on their own..!? Any ideas?