Snagit 10, Snagit 2018 on Windows 10 64-bit suddenly got very very slow several weeks ago. Slow to startup, slow to popup capture window, slow everything. It is not display driver. So many people are complaining about this. Did you change the call home routine?
Are you cleaning out your temp files? That will do it (amongst a lot of other things) Grab a copy of CCleaner or wise cleaner and run it. It is a good thing to do in general. Another issue is Snagit hogging resources which was supposed to be fixed in 2018 Open the task manager and find snag it and see if it is using a lot of resources. If so right click it and get the name of the file. Shut down allthe snagit entries so nothing is running, go find the file and rename it to something. THen start snagit snd see if the issue is fixed