That's not the problem, most DE file managers use an external process too (via dbus), but it's still slow, even on an SSD or when the cache has alredy been generated.

They're also buggy, XFCEs thumbnailer thumblerd (also used in LXDE and probably others) used to have memory leaks when it encountered video files with unknown codecs, newer versions re-scan the entire cache every time you delete/move a directory, making quite a bit of disk IO.

I've found simpler file managers like rox-filer, pcmanfm to be much faster at thumbnailing.