Domas documented a few things we needed to fix to make MySQL faster at accepting connections. Someone new to the team, Yongqiang He, spent a few weeks implementing some of the suggestions and the results are great. The tests used one 24-core host for the clients and another for mysqld. The clients were split across 8 sysbench processes and used sysbench modified to only do connections. The tests were run in two configurations with thread_cache_size=2000 and then with thread_cache_size=0.
Results with thread_cache_size=2000
Results with thread_cache_size=0