If it's 3rd party SSD, have you enabled TRIM? I had to do that for my old Mac Mini, made big difference. (2015 MBP of course has factory-installed SSD, but maybe this helps someone else.)

http://osxdaily.com/2015/10/29/use-trimforce-trim-ssd-mac-os...

It's all original Apple hardware.

It can be Spotlight reindexing, or Time Machine creating a backup into /Volumes/MobileBackups (done once per hour). Especially taxing if you have lots of files on your disk.

It's kind of annoying that these processes aren't niced and ioniced (or the macos equivalent). Especially when typing a password.

Look for hung mds (spotlight indexer) threads. They can get hung up and fail Un-gracefully with some files.

I had a client whose app generate PDFs that would cause this to happen.

Memory (8GB) is just suddenly jumping to red, usually by some Safari tab (gmail etc) which suddenly jumps to 1GB. And that's it. No indexing or mds processes; just browser tabs which suddenly jump over a point and lock up everything because of memory use. It's like i'm back in the early '90s when I first touched a non Amiga and non SunOS system. And asked how people can work with that 'Windows/DOS stuff'. I am not sure why it misbehaves so much though...