Which PSP can handle high transaction volumes well and doesn't have overly expensive fees?

I'm currently looking for a new PSP. I'm facing the same old problem: whenever there's a big promotion or campaign, sales skyrocket, and the system starts to slow down, sometimes even crashing. By the time it's recovered, customers have already left. So I'd like to ask friends, brothers, and sisters in the group if anyone has a PSP they're impressed with, especially regarding stability, handling a large number of transactions without the system going down easily, and reasonable fees. I don't want to pay too much, but I want good quality. Anyone with direct experience, please recommend. Thanks in advance.