Incredibly slow user interface and incredibly high disk writes
It's probably Dropbox or Google Drive. I've seen similar problems. Typically it's caused by Dropbox or Google Drive repeatedly downloading the same files over-and-over. They are both somewhat buggy programs.

Do you have any symbolic links in either your Dropbox or Google Drive? Neither deal well with symlinks; Dropbox specifically does not support them. (See https://www.dropboxforum.com/t5/Files-folders/Symbolic-link-on-Mac/td-p/27496 and https://help.dropbox.com/installs-integrations/sync-uploads/symlinks).

You can find symbolic links from the Terminal with this command:

find $HOME -type -l -print


or to see where it points:

find $HOME -type -l -ls


You can also run a program that monitors all writes to your hard drive by file, to diagnose the problem youself. Turn off System Integrity Protection and run opensnoop or iofileb.d.