I noticed a big framedrop when I switched to 14w28a to something like 15 FPS average, but even worse on 14w28b where I'd get an average of 3 FPS.
For reference I'd get stable 80 FPS on previous versions.
CPU and Memory usage is about the same.
Environment
Pentium G3220, 8 GB RAM DDR3 1333 MHz dual-channel, WD caviar Blue, R7 250. Windows 7 Ultimate 64 bit, Java 7 64 bits (update 55). Allocated a maximum of 5 GB for MC.
Fixed for a future version. MC-61451