Minecraft 14w26c, Minecraft 14w29b
Java 8u5 / 7u55 64 bit
Since version around ~1.7.5 Minecraft became sluggish and freezes randomly all the time.
edit: it actually isn't that bad on 1.7.10 - so now only snapshots are problematic
It's probably due to too many temporary objects being destroyed and garbage collected... Even if garbage collector manages to do it's job on time, it's not good as CPU usage is 100% all the time. (and it's noot good)
Using skyblock map was just a personal preference here. The same thing happens with "normal" minecraft maps.
No mods are installed. (and actually, optifine solves the issue for 1.7.10 version)