I faced a similar situation. Im not surprised to see an Incremental refresh being slower than Full refresh. For an ever growing data set, better to go for a Full refresh.

For an Incremental refresh, Tableau will first do a row-level comparison before deciding whether to import a row or not. Full refresh means it blindly grabs everything.

In ur case ur to import 120k new rows (which is just 0.2% of existing data) some extra time is consumed in comparing & skipping 99.8% of existing rows...

What kind of hardware do you have? I'm wondering if your hardware is too slammed to refresh quickly.