Typically a computer won't slow down because of a power supply issue. It doesn't know how much "juice" a power supply is capable of delivering. If the power supply can't deliver power, it will just shut down hard with a crash or power off. Seasonics are really good supplies. So it's likely it would just hard click off. (Not slow the CPU down however) So I think we can safely rule out your power supply.
That said, I would look to see if some other component in your system is responsible. I would check the temps and cpu clocks and see if they are hitting where they are supposed to hit. If you don't know how to do this, we can walk you through it.
It is possible you have reset the UEFI BIOS because of the power outage (SUPER RARE, but it does happen.) This would sometimes cause the wrong clock multiplier or memory timings to be set
It's also possible you knocked a CPU cooler loose by accident when swapping your power supply. An overheating CPU or GPU would throttle down.
I would also run a full virus scan and check your task manager to see if any processes are hogging CPU Cycles.
Lastly (and I hope not) a power surge might have damaged your computer. Again, it's doubtful. But I have seen power surges make computers flakey after the power comes back on.