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The big and immediate one was the engine was software rendered and optimized like shit compared to 2k(3). Like Hex said XP has it's own host of problems. It was one of the big points of frustration with VX that they threw it out for VX's horrible shoehorned in mapping style (along with a lack of multiple tilesets, VX's mapping features are fucking dire). RMXP's mapping is a direct upgrade of 2k(3)'s: There's three mapping layers instead of two (or 4vs3 if you want to count events), tileset restraints were basically thrown out allowing for stupidly big tilesets, iirc no tile is locked to a certain layer, and it supported alpha channels over a marked palette index as transparent.
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