Users are saying between 5-10% increase in speed, although I think thats becasue of the way you can set how many blocks are run before it reports back.

"Windows Beta Client [v2.5.08]

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This is just a stand alone exe. It's not an installer.

Caveats (for v2.5.08): Fixed a bug prohibiting entering a long username. Fixed a bug in the GUI that was cutting off 'test' in the config screen.

Caveats (for v2.5.07): The ones below still apply. This version has support for changing how many intermediate blocks between server reports, similar to the other clients. Under the config button, you will now see a place where you can set the amount of blocks next to 'Trans Int. Blocks' where it says "Freq:"

Caveats (for v2.5.06): YMMV. This is even more beta. This uses updated math libraries which should result in a speed improvement. I have not done calculations to figure out how much faster it is, but if other people would like to that'd be great. Because of the updated libraries, I need to do more testing on this client to make sure things are reported properly. Please test with it, but don't deploy widely"

http://www.free-dc.org/forum/showthread.php?t=13812

There are other betas for other OS's up a level in that sub-forum.