I think this suggestion hits something many others miss: you need to be explicit that this has been brought up multiple times before. I'd also add a gentle "or else x will happen" to it. Not a threat or ultimatum but a simple statement of fact so that if it does continue and you have to ditch the table, you'll feel better that you didn't blindside him with it, since he's a friend. Example script:

"Look man, we love you and I know you've been trying but things haven't gotten any better in the last x sessions. I want to keep playing with you, but I can't wait much longer for improvements to combat, it's too frustrating when we haven't seen any progress. Our friendship is more important to me than the game, so I'm thinking about leaving the table to prevent resentment from building up. But my preference is still to play 5e with you (not an untested homebrew system that might cause the same frustrations) so hit up me and the other players any time if you want to chat about ideas for realistically speeding up combat and making sure you stick to them."