Have you ever wondered about modern fighter jet speed? An assumption by many, "is the faster a fighter jet goes the more dominant it becomes as a fighter". In some cases that may be true but in other cases it may be a useless feature because the battle energy required comes from a slower speed in close quarters.
The F-35 development has startled many observer of having a top speed of only Mach 1.6 which falls below the faster breed of fighter going Mach 1.8 and higher with 2+. The answer may come from battle space efficiency, stealth and performance. What the F-35 gives up from being Mach 1.6 it will add on from being a far more efficient and invisible aircraft. In other words going fast would delimit the F-35's capabilities for which it was designed.
There may be a different assumption in play having the F-22 combined with having a F-35 commonality. The F-22 is the muscle where the F-35 is the brains when flying in formation. A flying formation of two F-22's combined with four F-35, communicates a need for further F-22's on order to complete with intense and purpose of total aerial combat.
The congress is looking to upload another 200 F-22's when money is found. If completed, then the F-35 becomes more deadly than its first take would suggest. It won't need to go Mach 1.8 as the F-22 goes Mach 2.0+ with stealth capabilities. The F-35 computers hook-up with the F-22 systems and sends it into combat with an autonomous pilot at the controls. Both the F-35 and F-22 become a greater force.
I would suggest that is the military thinking of synergism of both systems with each other. The F-35 is in its sweet spot at Mach 1.6 where its stealth makes it stay within its optimal capabilities of design and it can now operate from its most capable position given its multi functions. In other words, the balance between brains and muscle makes the F-35 a formidable and untouchable aircraft. Having the F-22 as its wing man makes the F-35 unreachable. No other nation has this type of one-two punch in its own formations. The F-35 is the screw driver and the F-22 is the hammer.