Since there are a lot of new players in the game due to sale, we've decided not to scare them off with an unpolished build, so we've rolled back to v0.5.1, which runs smoothly.
The performance problems in the v0.6 are not solved yet and won't be for some time, because it's a very complicated issue. Very sorry for that! To implement game state recording for goal replays we needed to turn parts of our code upside-down. These changes were needed for the online multi-player support and more such changes will be made in future. And it doesn't matter what GPU you have, this issue is unrelated to GPU performance.
The latest version 0.6.2 is still available in the BETA branch on Steam, you can switch to it freely, no access code required. Now you have a choice.
Very sorry for the inconvenience!
http://store.steampowered.com/app/399820/