Wondering the same thing myself, and yes sqlite3_column_type() by itself doesn't seem expensive.   I assumed in general it was to allow more responsiveness for apps with huge number of columns (i.e. large tuple size).  But we have about 20-25 columns and so I was going to try removing it and seeing the
results. In any case, it seems, fewer GIL acquire/releases  will help with throughput.  
Are there any guidelines on when GIL should be released?

Re PyTuple_SET_ITEM...yes that's also a possibility but it would then hide genuine bugs.