I know You're an expert and I was surprised that You can have any problem with iGO at all.
I believe that all possible combinations with sys.txt have been tested already with no success (really no need to specify them).
So only reason for this lag, I can think of, could be that iGO engine is not optimized for high resolutions and for modern multicore processors.
When it was developed resolutions were much smaller and processors were not so sophisticated as today and it was developed for resolutions up Full HD max (similar to old 16 and 32 bit programs working on new 64 bit Windows and new hardware).
As it seems to me, we will have to wait for newer and redesigned program version with modern engine as most of modifications and improvements until now were only functional and cosmetic.
I hope that iGO team is already working on new engine and that we will see it soon.
Sorry, I know that it was not helpful, but that's my opinion on subject.