How many times have you restarted NX since it initially crashed? What caused the crash (ie, what were you doing when it crashed)? Have you looked at your temp file directory and emptied it since the crash? There are a plethora of things that could be causing this slowness - it could just be the restart and once you get to using it again, things go back to normal.
Also, try deleting the "DialogMemory.dlx" file from your {user}\AddData\Local\Siemens\{NXversion} folder. If this file gets corrupted or grows too large, it can slow down NX. When you delete the file, NX will create a new one the next time you use NX.
There are also NX settings that are saved in the registry under HKEY_CURRENT_USER\Software\Unigraphics Solutions\NX\. Deleting that folder has fixed weird UI and NX behavior issues for some of my users.