Rafale's speed is indeed Mach 1.8 as given in Dassault's official site itself. It certainly cannot do more than that, else the frenchies would be trumpeting that all over the place to say they can match the Eurofigher in that regard. MKI's max speed is only Mach 1.8 ~ 1.9.
Speed is important because of interception. While this is very important in war, Speed is extremely important in peacetime perhaps more than any other capability of a fighter. This is because during peacetime a fighter has to visually ID the intruder and give them verbal warning before shooting it down if it doesn't obey. For this it has to take-off, climb fast, reach its max speed, get to the intruder's co-ordinates fast all in a few minutes before the intruder slips back into its airspace. So Timing and Speed are of THE essence here. During peace time one does not fire BVR missiles at any object which slips across the border. Visual identification is a international practice, since the intruder could very well be a lost cessna or any other non-military aircraft. In these circumstances, Speed is THE crucial factor since it has to get to the intruder's location fast. The Atlantic shoortdown was only possible because of the MiG-21s scramble-intercept capability. But not to worry, even after the 55 odd MiG-21FLs were retired a few days back which were primarily tasked with fighter training and pilot conversion duties and not active frontline squadrons, IAF still has 90 odd MiG-21M/MFs as primary ground attack and secondary air-intercept duties, around 50 MiG-21Bis as primary air-intercept and secondary ground attack, and another 105 MiG-21 Bisons with air-intercept, Dogfighting, Ground attack.. quasi multirole capability. That's still about 250 MiG-21s, making it still the largest type in the IAF inventory!
The fastest plane in the IAF inventory after the retirement of the MiG-25s are the MiG-29s with Mach 2.3, followed closely by the Mirage at Mach 2.2.
