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).

The star wars universe as a whole is supposed to be in the dark ages after the glory and progress during the High Republic they are stuck/regressed, which is why everything looks like it's cobbled together