How do I know when my HD is under maintenance?

I work with several large movie files, almost all with 150+ GB. And sometimes I experience a harsh slow down on my iMac's speed which is accompanied by a writing/reading HD noise. So I'm pretty sure the disc is defragging or doing some kind of maintenance. According to this article: https://www.macworld.co.uk/how-to/mac/how-defrag-mac-why-you-dont-need-3600241/ "That's not to say that a manual defrag is never needed; it's just rare. People (usually creatives) that have hundreds of films/audio/multimedia files larger than 1GB may need to defrag a Mac." So I wonder if: **is there a method to find out if my HD is under maintenance?** Can I took control over this process? Like changing the schedule of the task, maybe using one of the listed tps of the article? ---------- Further info