When playing on snapshot 14w25a my FPS much lower than normal. I used to 90 FPS(In Minecraft 1.7.9) and now I get 20 FPS(14w25a). I have been switching between the two versions and get the same results every time. I am playing on max settings if that helps.
Mac Pro (Early 2009)
• Processor: 2 x 2.26 GHz Quad-Core Intel Xeon
• Memory: 12 GB 1066 MHz DDR3 ECC
• Graphics: ATI Radeon HD 4870 1024 MB
• Software: OS X 10.9.3 (13D65)
Java is the latest version as of June 18, 2014