Will purchasing an NVMe SSD noticably improve performance

[odinson]

Did you manage to successfully install both of those on the PBP? systemd through up errors for both of them for me:

Okay, I got the errors too on install and iotop won't launch, but atop works for me. See if you can use that and look for the "DSK" row. If, when you get the phantom slow down, you see the busy % showing a lot of activity, that's indicative of it waiting for disk i/o and that points to a high probability that an SSD would help with the slowdowns you're experiencing.