There's a few accusations of Piastri slowing down, but those claims are made by people who don't understand the delta times. In F1 you do not keep a constant speed under a VSC (unlike other series which have these rules - F1 is more complex). In F1 each mini sector gets a delta time you must be slower than, rather than just keeping everyone at 80kmh. That means the cars speed up and slow down, which is what Piastri was doing. By the end of the VSC it will have leveled itself out.
And that's exactly what we saw. The onboard from Max shows him the same distance from Lando at the green as he was when they went under VSC.
Max was penalized because he tried to pre-empt the restart (all drivers and teams do this and have systems in place), but went too early which took him under the delta time for that mini sector.
The penalty isn't really one you can contest. You're either above the time or below the time - it's just a factual one. Normally, the penalty is 10 seconds, but I don't think Max did it on purpose. He backed out of the pass before the corner. I think he had his penalty reduced because of that. I think that's fair.