External miner
Both miners are not yet optimized, but the built-in wallet miner is even worse, it does not use the GMP library which should give it a big boost.
Have you done any work on it yet? I might try to this weekend. I can see a number of improvements.
I'm giving it one last try using a single RPC server for all of my farm machines now to see if I get any blocks. Time to block is around 14 minutes with my farm so we will soon see.