To fix SSD runs slow on macOS, you should back up your drive and recover data from it first, because your drive may have a risk of losing data.
And it’s highly recommended to utilize iBoysoft Data Recovery for Mac, which can recover data quickly from both SSD and HDD.
Then you can:
- Repair the drive with the First Aid in Disk Utility.
- Quit some apps using the external drive.
- Update macOS…