Yep, same problem here. I'm also seeing delays of 30 to 60 seconds or more just clicking on the IDE to do anything.
Have you checked the memory available (RAM) ?
Did you have a look in instruments, to see which resources are most limiting (CPU ? RAM ? Disk ?)
You might want to try using the Legacy Build System (which was the build system by default in Xcode 9).
For our project which is very large and has a mix of Obj-C and Swift the new build system is about 4 or 5 times slower and fails to ever build incrementally.
You can change it in File -> Workspace Settings.
The same thing happened to me two days ago, I tried to fix it by uninstall and reinstall Xcode 2-3 times, but no cure!
Finally, I uninstalled Xcode, reinstalled the MacOS, and installed the Xcode, it was good that MacOS kept everything in place, so I just installed Xcode again, also had to:
$ xcode-select --install
In bash again to bring my Git back to life.
Then I opened my project. The first opening and compile took a lot of time, about 5-6 minutes, to refresh indexes and other stuff; but after that, the slowness problem solved.