InnoDB takes a lot of time to perform null updates. The reason is that
even though an empty update vector was created, InnoDB will go on to
write undo log records and update the system columns
DB_TRX_ID and DB_ROLL_PTR in the clustered index, and of course write
redo log for all this.

This could have been fixed properly in
commit 54a492ec more than 10 years ago.