We know Solo made some "special modifications" to the Millennium Falcon, but how much faster does it go compared to an (original, "off the shelf") YT-1300?
The Starlog Technical Journal (Part 1) indicates that the stock YT-1300 only had a standard light-speed plus point one hyperdrive as opposed to the souped-up "light-speed plus point five hyperdrive" that Han boasts of in Star Wars: A New Hope. Quite what the speed differential actually is isn't explained in any great detail.
The Millennium Falcon's power, propulsion and computer network arrangements require a constant maintenance schedule that keeps her repair and adjustment time at a level nearly twice the ship's flying time. Solo regards the trade-off as well worth it, for more than once his life has been saved by his ship's speed and extreme manoeuvrability. A beefed-up hyperdrive system is at the core of the Falcon's increased speed, which is well above the light-speed plus point one capability of the standard Corellian YT-1300. Four fuel slug tanks in the ship's power core give her an increased range, even at extreme speeds and engine temperatures. While repairs to her various systems must usually be performed each time the ship lands, she refuels at an average rate of only once per month.
The Millennium Falcon Owner's Workshop Manual factbook offers the following comparison.