No amount of momentum can stop fatigue. And however skilled you are, it will kick in. And when it kicks in, you will bowl more poor deliveries so will be more likely to concede runs. With a stronger attack you'd have bowled less so if you took 3-40 in your opening spell you might have not bowled again rather than bowling again and ending-up with 6-150.
First of all, the differences are not much so fatigue is really not much of an argument. Again, momentum plays a big role. You speak as if you have never played cricket. Do you think someone who is picking up wickets and hurting the opposition tires in the manner to completely degenerate? Not at all.
Eh? What's that to do with anything? If you both bowl the same number of overs the strength of your attack hasn't impacted upon your over count, has it?
Yes, it has everything to do with it. Because you have to bowl much better to outdo your teammates than you would if your teammates were much weaker. Both bowlers can take 4/100 but one bowler had to be much better because of the wicket-taking competition his teammates force on him.
Assuming you can numerical measure performance, I'll give you an example.
Let's say team A is the stronger wicket-taking team and team B the weaker one.
If you play in team A you need to bowl at 80% to take 4 wickets because the rest of the attack are also bowling at a high standard and will take those wickets if you don't. Whilst in team B you can take the same amount of wickets bowling at 60% because the wicket-taking threat is simply not like team A's.
And there's no finite number of runs you can concede, so even if you bowl more overs and get a few more wickets, you're going to concede more runs, and the more overs you bowl the chances of you conceding more per over goes up.
No, but if you bowl roughly the same overs you will concede roughly the same amount of runs but you are more likely to take wickets because of your support.
And if you are not taking wickets and you keep conceding runs then that is what you term "a bad performance" which as I said in the beginning, is not common like good performances are with "great" players.