If you turn on Preferences / Miscellaneous / Advanced: context_menu_debug, then run DebugView and right-click the file that's triggering the problem, it may be a quick way to reveal the cause.
(DebugView is a small Microsoft tool; no install required, just unzip and run.)
The debug output will list each part of the menu as it is built, and will also list each shell extension individually. If there is a long delay between two parts or extensions, the one just before the delay is probably involved in what's going wrong. (Sometimes it ends up more complicated than that, but it's a good place to start.)
There's a bit more detail on this approach in the Crash, exit or high CPU when right-clicking certain files guide's Finding the Culprit section. (The guide is about crashes but the technique can help with slowness as well.)