Problems freezes on MacBook Pro

I have an Early 2015 MBP and it's worked perfectly until yesterday. Yesterday I was editing a movie in iMovie and suddenly iMovie started getting choppy and would take around a minute to perform one command.

I first thought it was because I'm running my iMovie library from an external HDD, but I haven't had problems with it until now. I tried moving the library back to the SSD of my MB but the same problem occurred. I checked the Activity Monitor and it says that 70% of the CPU was idle while these freezes were occurring. I tried restarting the MB and it would work smoothly for about 10 minutes before freezing again.

I saw a discussion (https://discussions.apple.com/thread/3852075?page=2) about the same problem and there I read that the solution was deleting a preference file for iMovie. But this, and other solutions, didn't work for me.

Later the same thing started to happen while watching YouTube videos on safari. It would work smoothly for around 15 min before all videos I would load would just be buffering, but never starting. I would get a message saying "Having problems loading the video? Try restarting your device". And I'm pretty sure it's not an internet problem considering youtube works perfectly on my iPhone and other devices.

I'm worried that it's not a problem with safari or iMovie. I think the problem lies with the MacBook. And I'm not sure what to do. I'm planning on maybe factory resetting the computer. But I'd rather not. If you have any ideas on how to fix this, please let me know.

P.S. I'm running:

MacBook Pro Early 2015

13"

2,7GHz i5 Processor

8GB RAM

128GB SSD

macOS Mojave (10.14.4)