What could cause "lagging" in SSIS execute SQL task?
Chances are your drop/create statements are blocking each other on occasion.  As already mentioned, you can truncate the tables instead, however a fairly simple test would be to run the drop/create tasks sequentially, and the data loads afterwards in parallel.

If you're desperate to know the detailed reasons behind the variable times, then do as suggested and run a trace or XE session to capture locking &amp; blocking during the runs.