Dataflow: On-Prem execution not supported for entity - Can't Merge Two Tables with Dataflow?

Sorry for the  delay, thank you for your reply Winniz! I was already aware of the ability to "not load" the merge tables, though I didn't know that I could make these Dataflows without BI Premium. That being said, I have played around more with the Dataflows I am building, and I have discovered a few things:

1.) When I convert my Dataflow to make all "connected" tables NOT load, I am able to get my On-Prem tables to load. However, this still doesn't answer the question as to why I can't merge tables that are On-Prem, because I DO currently have a Premium license, and I merge tables in another Dataflow that is connected to our CRM (Dynamics 365) without issue. Is there NO way to merge On-Prem tables without disabling load on the connected tables?

2.) I liked the idea of converting things since if I have to avoid merging tables anyway, why even bother with the Premium license. However, the same Dataflows when run in a Pro environment vs Premium are MUCH SLOWER. I can refresh with a Premium workspace in under 15 minutes, but the same Dataflow takes 1 to 2 hours when run under Pro. The Dataflow is EXACTLY the same, I literally pulled a JSON file and used it to create the Pro version, it just seems like the Pro version is significantly slower. Could this be beause I don't get the Enhanced Computation Engine when I am on Pro? I don't mind paying for Premium, it just feels like I should be able to do more with my On-Prem tables with the Premium license.

3.) My idea of just duplicating tables so I could use one for a merge that DOESN'T load, then have one that isn't used for a merge but DOES load seems to work, though it of course slows down the overall Dataflow. Wish I had a workaround, but I am guessing with On-Prem that I am just stuck, unless I am missing something.

All in all, I am fine with the current setup, more just disappointed that we are going to pay double the licensing fees for BI just to get the system to refresh faster. It is much faster, so I can't complain that much, but I just wish someone could explain the limitation on merging On-Prem tables. Is there a real reason this can't happen from a technical standpoint, or is it just not supported?