Did task manager show Squirrel (the seperate updater) running in the background even though Discord wasn't open?
I'm thinking that it may still have been updating Discord.
If your computer is working as normal, slows down, then after a while goes back to normal that is most likely a sign that something is updating in the background.
Windows Defender definitions can often be the culprit there, they update at least daily.
Sometimes a slow boot is because Windows Update is updating things in the background when you boot.
Often Defender definitions again, but you will also see it lasting much longer when you first boot or restart following every second Tuesday of each month - This week was the second Tuesday in the month.
That's because Microsoft push out Windows 10 updates on that Tuesday, which download on Tuesday or Wednesday and then install next time you start the computer thus slowing it down for a while.
Browsers such as Chrome and Firefox, and other softwares also push out monthly updates at the same time.
As well as updates:
Restart will always take longer than Shutdown/Boot with Windows 10, it's the way Windows 10 works.
Shutdown will always hibernate the Windows Kernel so that it can then boot faster.
(That's a different thing from you selecting 'Hibernate' which hibernates everything that you have open).
Restart does not hibernate the Kernel and so starts Windows completely from scratch, which takes longer.
You should do a restart regularly, to clear things out. Windows Update will usually make you do a Restart at least once a month, but some recommend once a day is better.
(It's also why anyone helping with a problem will tell you to do a Restart and not a Shutdown - Restart clears more in Windows 10).
If background updates or processes are really slowing you down then it may be time to consider updating your computer or getting a new one.
Get more RAM in there if you can, get a bigger Hard Drive or better yet get a Solid State Drive (SSD).
I'd also suggest running ADWcleaner from Malwarebytes, it can remove unwanted/uneeded apps that are slowing things down, use with care and check what it saying can be removed (you might actually want/use some of it).
https://www.malwarebytes.com/adwcleaner