exported synced sequence hardly plays in FCPX. The program even tends to not respond and crash

I have problems with the synced project after importing the xml into FCP X. I even updated my system to catalina and I have the latest version of fcp x.
The video material is from the sony fx7, so it´s mxf files.
I can see the timeline and the synced clips, but while playback the programm tendss to not respond anymore. Does anyone have an idea what I can do?