SSE2 has nothing todo with multithreading or 128bit data types, its actually just a way to do arithmetic really fast by using magic known as SIMD (single instruction, multiple data). This is useful if you have multiple numbers you want to do the same calculation with, for example rows in a matrix when doing matrix multiplication.

And by meta data I mean something like not only storing the actual objects as eg 16 bit numbers in one large row, but instead write some more information together with it. Look at what data you have and search for common patterns and then a way to express this inside your serialized data.
Lets say your world follows this pattern: 01010101010101010101 (where 0 is a tree and 1 is a patch of grass or whatever you want), then you could store the information that there are 20 zeroes and ones alternating each other. Of course this makes only sense for often repeating patterns, and isn't really worth if something like this occurs only a few times.