Modular Rocket Systems is a parts pack for KSP, with the following design goals:

Fill in some of the gaps in the selection of stock parts. For example, size-3 and size-0 SAS modules, and size-3 monopropellant tanks. Preserve the stock balance. All parts have their costs, masses, and capacities chosen based on stock analogs. Provide some additional cosmetic choices. Examples include the 2.5m fuel tanks, and various nose cones. They have direct analogs in the stock line-up, and share the same stats, but give you additional aesthetic options. Maintain a stock-like appearance and seamlessly fit into the game, while following its own artistic style. This one is trickier, but by taking cues from the stock-parts, it's possible to make something new, that also fits with the game's existing aesthetic. Be friendly to your CPU/GPU. Very importantly, these parts follow most of the stock-part rules for how polygons are used, and collision meshes are configured. They shouldn't reduce your framerate any more than the stock parts do.