Summarizing above replies -
a) Even though an incremental refresh is set up - the database query will remain unaffected and hence the execution time would remain about the same from a database perspective
b) Incremental refresh has a limitation that the dataset will eventually hit the 1Tb or 1 billion record so the customer has to account for this when setting up the refresh.
For B- is there a plan to address this feature request Remove Old Data During Refresh