mdsync at 99% CPU

First, have you tried restarting the computer?

Are you running any anti-virus or so-called "cleaning" apps?

Processes starting with "md" are unsualy related to Spotlight indexing. Sometimes the default settings for Spotlight have it indexing unnecessary items.

Change Spotlight preferences on Mac - Apple Support

Example: if you have Spotlight indexing your external drives, which are slower than the internal drive, the process can take much longer.

Personally, I omit emails, calendar events, and similar items from indexing because I can search from the client software on the rar occasions I need to find something. Be thrift in what you have Spotlight looking up.

mdsync is a system process that is related to Spotlight. In turn, Spotlight becomes very active during the Time Machine backup process. This is one area that is consuming CPU cycles.

FWIW, if you are using Activity Monitor to get that "99%" value, you should know that number represents a single core of that CPU ... not the entire number of cores on that same CPU.

The other value to monitor, in Activity Monitor, would be System. This would represent the "stress level" for all processes run on the CPU.