Why does replication being behind, affect the performance of our customization that uses ROQL?

Environment: 

Customization, ROQL

Issue:

I have a customization in place that is utilizing ROQL and when replication is behind, it affects the performance of this customization.  Why is this? 

Resolution: 

ROQL queries always run off of the Production database unless a query has been blocklisted.  If a query is blocklisted, it will either be blocked or it will be redirected to run off of the Replication database.  There may be times when the Replication server falls behind which may affect your ROQL customization if the query is redirected to run off of this particular server. 

Most blocklisted queries can be redesigned to be compliant.  ROQL queries are also run by product functionality behind the scenes, and those queries can be blocklisted as well.  If blocklisted queries are suspected, please submit an SR to Technical Support for further investigation.