I've noticed this from time to time and it may be just how the net is acting that day especially if you regularly keep your system clean with an app like Ccleaner.
Just to make sure nothing nasty is causing the slowdown try running this app from time to time too as it's very good at removing malware as it's easy for it to sneak onto the system: https://www.malwarebytes.org/
Oh right gotcha... If you go into advanced options you'll see a checkbox to not delete anything downloaded in the last 24hrs I always uncheck that too. Also try running Ccleaner as administrator too.