Linux Steam beta is slow

[abelthorne]

When playing Hotline Miami with the current beta build on Steam Linux, the game is really slower than expected (it feels it runs around 50% of its normal speed). When playing the non-Steam version from the Humble Indie Bundle (v. 1.0.9a), it runs fine.

So, the problem seems to come from the current beta build (I've tried to disable the Steam overlay just in case, with no change).

I have an ATI GPU. From other players I know on Linux too, it seems they have no problem on Intel or nVidia GPUs so it might be a bug specific to ATI cards?

Specs:
Ubuntu 13.04 64 bits with Unity interface
ATI RadeonHD 7770 with driver from the Ubuntu repos (tried to update to a recent beta driver, it didn't change anything and was quite unstable).