To get 23-27MBps you must have all gigabit ethernet and 'n' or 'ac' Wi-Fi. My theory is that one device or link is not performing correctly. See if you can find that element.
You said that the new router should be working properly, but you need to confirm it.. Can you try a transfer by ethernet with it in the path and then again with it out of the path (either skip it or temporarily replace it with another switch)?
You said that the new router should be working properly, but you need to confirm it.. Can you try a transfer by ethernet with it in the path and then again with it out of the path (either skip it or temporarily replace it with another switch)?