Mojang team finally managed to locate and fix the reported performance issues of 1.5, so now it should be at least similar to the performance of 1.4.7. Mojang team also fixed a number of bugs (as seen in the snapshot release last week), and a couple more. The 1.5.1 patch notes are a bit simplified, though?
Mojang team intend to publish the release on Thursday, unless something nasty happens, or Searge of MCP asks us to wait for them to catch up.
Credits: The Minecraft and Minecraft Realms teams