Unusual behavior after downloading Stuffit Expander

Try uninstalling it & see if things return to normal again. From what I have been able to find out Stuffit Expander is essentially a Mac program although there is a Windows version.

As Ranger4 suggested: Uninstall it for now.

If you want a light weight solution (very old, tried and true): Download 7-zip instead.
It's supported on practically every type of hardware out there and every operating system you can think of.

I'm not trying to advertise, but! 7-Zip is a REALLY good piece of compression/decompression freeware software!

debodun: As far as the lag goes: I'm assuming it's due to the HDD being taxed by something (heavily).
If Windows Updates is left on automatic: It could be installing whatever update(s) in the background (coincidence).
It could be any program that has some form of 'automatic updates' enabled. It could even be the Task Scheduler set up with an automatic defragmentation period.

If you want to check what's starting up at boot-up time (not an all inclusive list) using native Windows tools:
- Press Windows key + R (opens "run")
- Type in: msconfig
- Navigate to the: Startup tab
- Navigate to the: Services tab
- Tick off the "Hide all Microsoft services" (quick filter)

A program known as: CCleaner by Piriform will accomplish the same tasks (and more) with a more modernized and easy-to-use interface.