After upgrading Windows 7 to Windows 10, Mathematica 10.2.0 developed this same problem. I reset the configuration as suggested and it helped reduce some of the excess freezing and crashing, but the "Formatting Notebook Contents" message still appears often and there are long delays when doing certain things. The autocomplete help sometimes has a long delay, causing the notebook interface to freeze until the list of matching commands pops up. Saving a file and closing a file takes a long time. Etc.
BTW, I noticed that the main
Mathematica.exe process is spawning 2 instances of
WorframKernel.exe. Is that normal?