Validation is pretty slow for larger sheets. Look into cutting down the number of Google API calls -- these are what's slowing it down. Profile via View -> Execution transcript.
[x] Google recommends against importing third-party Apps Script libraries because that process is currently slow. Instead they recommend copying the library code into your add-on. It's worth investigating the performance gains of doing this for the SheetConverter library (it's BSD licensed).
Validation is pretty slow for larger sheets. Look into cutting down the number of Google API calls -- these are what's slowing it down. Profile via
View -> Execution transcript
.