Most sites run as well, if not better, on Firefox for me.

If you're running a quick and dirty test, you might not get an accurate picture of the performance differences. For example:

• Your usual browser might have cached some content from the last time you used it
• Unless you kill them properly, your computer might not have the RAM/processing to be running two browsers at the same time with the best performance
• One browser might be bogged down with extensions / issues that built up over time

You could try giving Firefox a clean install, or opening it in safe mode (it's now called troubleshoot mode), to see if there's any difference