Performance went down when battery completely died MacBook 13” mid 2014

I’ve had the computer 7 years. The battery life ended at this time and I was running off just plugged in power. When on just plugged in power the performance was severely impacted and I’ve is extremely slow. I went to the apple store and they assured me a new battery wouldn’t fix the issue. I got a new one put in and it fixed the issue. I had it removed because I don’t have the money needed for the battery. I asked them how to fix the performance issue without the new battery and they couldn’t answer, hoping some others on here have found solutions? I’ve erased the whole computer, started in safe mode, and first aided the drives and the performance is still horrible.