Combining MXF files with MOV files in FCPX

Bob is right.

Select one of the mxf clips in the browser, control-click, Transcode Media and check the box for proxy.

Then in the little menu at the top right of the viewer choose “Proxy Preferred”.

Compare how the one clip you made proxy for plays back vs another.

https://support.apple.com/guide/final-cut-pro/create-optimized-and-proxy-files-verb8e5f6fd/10.5.1/mac/10.15.6

You can do it just for the files that benefit from it.

Since apparently it's only the mxf files that your mac has trouble playing back smoothly, you can select only those and convert to proxy.

That is the beauty of the "Proxy Preferred" option: if there is no proxy for a file, FCP will use the original.

Proxy are smaller and easier on your mac. Don’t worry if it looks of lower quality than your original because in the end you will switch back from Proxy Preferred to Optimized/Original and export in full glory.

Of course you can combine different types of video files.

What is your mac, where are the files located, how is the drive connected?

Does the problem occur in the browser as well as the timeline?

Does the frame rate of the clips and the project match?

proxy..proxy..proxy..

yes..