When discussing system CPU usage on a Mac, you cannot exit the conversation without discussing mdworker and mds. In Activity Monitor, this process will not appear unless you select all processes, and then, these processes seem to be ubiquitous and resource hogs. So, what are they?
Mdworker is short for Meta Data worker. This is a background process on your Mac that spends time indexing your files for Spotlight. Without mdworker, your Spotlight searches would be less than accurate at times. Mdworker returns its results to mds which builds your Spotlight database.
Why is it that it can be processor intensive? As it indexes your files, it must first read all of them. As it reads them, it then exchanges the information with mds to create the database.
So, how do you minimize mdworker and mds? You don’t. You could choose in System Preferences to stop indexing of all volumes in Spotlight and that would prevent these processes from being active. Often, you just have to let the processes run. Certainly, if you plug in a large external HD and do not want it indexed, or connect to a Time Capsule infrequently, you could disable the process from searching these volumes — reducing their workload when those volumes are preset.
Ultimately though, you would not be able to use Spotlight for searching those external devices effectively in the future also. While mdworker and mds can seem to be processor-intensive processes, their benefits far outweigh their costs in the long term.