The F-35 isn't meant to be an observation platform, for which long endurance drones are a much better option, it's a strike fighter. It's supposed to be able to

The F-35 isn't meant to be an observation platform, for which long endurance drones are a much better option, it's a strike fighter. It's supposed to be able to use its stealth to fly through defenses other planes couldn't and deal crippling blows behind those lines. However stealth isn't invisibility, those defensive radars may not be able to lock onto the F-35, but they can see that it's there and scramble fighters to intercept. A big advantage of the F-35's stealth and advanced electronic suite is that it can see enemies before they can see it, and can avoid engagements where it is at a disadvantage - but this relies on it having the speed to get out of dodge before the enemy can close the gap. With limits on speed, it takes longer to complete strike missions which increases the risk of intercept, and the ability to avoid those intercepts is potentially compromised.