The millennium falcon has a fuck ton of modifications made to it that actually make it a good ship. The issue is that it's all slapped together redneck style so it's very unreliable. Han's claim that it's the fastest ship in the galaxy is not actually that far off. It has a Class 0.5 Hyperdrive, which is the fastest hyperdrive in both canon and legends (with a few very obscure and circumstance-dependant exceptions, like the Jabitha and Aing-tii).