Why is my iMac's CPU maxing out during simple tasks like music and file copying?

Without the benefit of viewing your computer directly, I'm inclined to think it's OK. I've noticed copying files increases system workload.

Check Activity monitor again after the transfer finished. If it has been a long time since you restarted your computer, please do so after the copying is done.

If you have ill-advisedly installed any anti-virus or so-called cleaning apps, they could be interfering with normal system functions.

It's not a bad idea to vacuum the vents to remove dust. It is not necessary to open the computer to vacuum (this is highly invasive and difficult/potentially dangerous to you and the computer to do). Please remember to NEVER EVER blow air into the computer!

Pertcev wrote:

Thanks, Allan! I'll do that. I never install anti-virus or cleaning programs.)

One more question.

Do you think it's necessary to periodically clean iMac from dust inside?

I am having different thoughts on cleaning the inside than before. I used to feel it was a necessary on a regular basis, but two things happened:

—Earlier this month I opened my retired 2012 Macbook Pro 13. The last time I opened it for a dust-bunny eviction was late 2018. Then, the only accumulation of stuff was on the heat sink the fan cools and it was mild after five years of travel in a fuzz-lined sleeve. Two weeks ago I opened it to add RAM and checked the heat sink at the same time. No more stuff there than in 2018. That computer has traveled far and, at home, lives in a rather dry and dusty climate.

—Realizing the hazards of opening a sealed case iMac, today I only gently clean the air intakes of my 2017 iMac 5K using a non-static brush and a vacuum nozzle held at least two inches from the vents. I never use "canned air." Improperly used, it can cause moisture to form on computer parts that are very moisture-averse.