That was lots of questions. I'll answer one. >I know to get the best speed, 0.4 is needed. Still, (for the above) what are >the problems for 0.3? Have most of the fixed speed issues been backported? Is >Compat.jl needed (or have anything to do with speed?) I think slicing and >threads stuff (and global?) may be the only exceptions.
We maintain 0.3 for obvious bug fixes, and try to be really careful to not introduce new ones. Most of the performance enhancements are deep changes to the system that might introduce bugs, so they can't be backported easily. Compat.jl is a tool you can use to support 0.4 and 0.3 with the same code base, without depreciation warnings. It has nothing to do with speed.
