This is why I asked if the media is external or managed.
If it’s managed, I’d try using fcpx to move to an external location. Does that go fast or slow?
If it goes fast, you can delete the media from the external location using Finder to help separate the issue from FCPX. When using Finder does deleting the media go fast or slow?
If the first process of moving to an external location is very slow, I’d start looking at the format of your hard drive. Is it os extended? FAT32? Encrypted?
A simple drive speed test like the likes of Aja system test or blackmagic desk speed test may also help to see overall drive speed.