I just upgraded to Mojave. Since the upgrade, my computer is unusable while Time Machine is running. The TM back-up isn't slow and it completes fine (backing up about 200-300MB most times). However, the rest of the computer becomes very slow. The cursor won't move, typing gives no text (and the text typed isn't just stored in a buffer but is lost), Clicking new windows won't change the focus, web pages won't load, and so on.
I am using an i7 2017 27" iMac with 1TB SSD internal drive. I have done searches but all Google, etc. reports are complaints about the TM back-up being slow, nothing about lack of responsiveness of the main computer during a TM back-up.
I wondered if there was some way to reduce the run priority of TM. Any ideas appreciated.