Why is Windows slow when disk usage is high?

I have always noticed that whenever disk usage is high (above 50%) the entire computer slows down, mainly explorer.

My question is, what is the reason behind this? Why is that when disk usage is high, performance is at its lowest? Does it have something to do with the HDD or is it an OS or CPU thing?

For example, I have some Node projects that aren't really needed anymore so I'm using rimraf to delete the node_modules directory, which contains thousands of files and directories. During the execution I notice that the disk usage is at about 50% and I honestly can't do anything during that time. I can't browse any other directory or right click a folder.

I have Windows 10 21H1 x64 which is installed in a separate drive (an SSD) from the files I'm deleting. I've also tried searching for an answer to this but most of the results are about fixing issues that cause 100% disk usage.

Solution 1:

A hard drive can only read one track (or several sequential tracks) at one time. If the head has to move to get the next logical track, it will take time.

5400-rpm drives are common in economy machines and are very slow compared to even 7200-rpm drives and certainly compared to SSD drives.

A way to speed up hard drive access if you can do it is (a) see if you can delete or back off many small files that slow down access and (b) set defragmentation to be automatic and run (say) weekly.

If you can convert to SSD, that will do a lot to alleviate your issue.