I’m working on a project with some stills. I scanned them at high resolutions cause I wasn’t sure what I’d zoom in on. The biggest ones have dimensions like 5028×3394 etc:Thus far, working with these hasn’t been wonky…and I read a Larry Jordan page from 2015 where he said FCP X wants sides less than 10,000 for size. So, I think I’m in the clear. But maybe I’m not….and I’d love for someone to tell me if I’m wrong. The program is often sluggish but I think that’s cause I’m working on two 10 TB drives and it’s a big project.
Make your stills the size you need them. You don't gain much from having them so large. And yes, very large stills, once you get enough of them in your Library, can slow things down. Unless you're editing 4K, no need for all that extra storage space.
depends as well on the codec used to create the image. Jpegs work fine. I have used jpgs 8k on a side without problems. PNG are significantly slower at the same size. TIFFs are slower yet. The slowest are PSD files with multiple layers and internal efx.