I had this problem and Sage Support told me to clear my temp folder.  Once I did, the problem was resolved.  However, you need to do this every time the Notes field starts to take a long time to open.  I now do this as habit at least once a week.  I believe the software looks at this folder to do a spell check or something else.  Once the temp folder is open, I delete everything in the folder unless Windows has it locked.  

To open the Temp folder:

I type "%temp%" without quotes in the Windows search bar at the bottom left of my screen in the task bar and press Enter to bring it up.  You can also use File Explorer to open the Temp folder too.

Hi brakeal. First make sure you are on the latest version. This was an issue with the real-time spellchecker in Notes, which we have fixed. If not, then in the Notes box, go to Options and uncheck the option to Spell Check While Typing