Array{Number} is an array of objects with unknown type and size, basically an array of pointers with possibly totally different method implementations. Array{:

Array{Number} is an array of objects with unknown type and size, basically an array of pointers with possibly totally different method implementations. Array{:<Number} is when LLVM gets the exact type at compile time and can optimize the code using SSE instructions. You have a factor of 10 between execution, and if you are not experienced, you don't understand why your code is slow and why the garbage collector kicks in so hard.