Why does BleachBit lag so much?

Is the lag about the same for preview and for delete?

Could you please try to narrow it down to a single checkbox? Two ways are: uncheck all boxes, and then check them one at a time. Alternatively, right click on the checkbox and choose preview from the context menu.

Do you have Steam installed or any Steam games? (Steam is a gaming platform.)

With this information, I can give you a more specific help.

1st clean took about 5hours. Second, I shut it down when it reached halfway after 16hours. [left it on overnight]
I'm a raw beginner and left most boxes ticked. I read the explanation of each box most of which I didn't understand. I think I'm way out of my depth & wondered if I could still use it if helped. !!

Please see Frequency Asked Questions: "Why does BleachBit take a long time (more than five minutes) and fill up by hard drive"

Here's a copy

A: You enabled the option to System - Free disk space to wipe free disk space for privacy. This works basically by creating a large, empty file. When the hard drive is full, the file will be deleted immediately, and there will be no net change in disk space (you will be back where you started). It is generally recommended you disable this option.