Performance - Insert to temporary table

I have a Repeat\Until loop that builds records and inserts them into a temporary table. The table's primary key is "Entry No." with the records being inserted in increasing "Entry No." order.

Creating and inserting 250 records takes about 2.5 minutes.  If I comment out the INSERT statement, but let all the record create\validate code run, the process only takes about 1 second to loop for the 250 records.

Any thoughts on where the performance bottleneck is?  And how it could be improved?

More info:

The table has no event subscribers, and is not tracked in change log. Change log is not even active.