Perhaps your having a contention problem, if the slowdown occurs while
Defender is not running a scan. What other real-time security programs (AV
and AS) do you have installed? You could try taking a look at Task Manager
(Cntrl+Alt+Delete) to see if any processes are noticeably burning up CPU
cycles. System Idle CPU should generally be in the upper 90s while nothing
else is going on in your system, meaning that there are plenty of spare
cycles left for processing. Also try turning off real-time protection
temporarily to test if that returns your system speed to normal.