Thanks! I've got it figured out, so I'll post what I know.

Basically, it's the same as overclocking in RPi OS.

Edit the config file with: nano /boot/config.txt

I used SSH to login to my box, but you could edit that file with a text editor on another system as well.

On my Raspberry Pi 400 I used the following:
over_voltage=6
arm_freq=2200
gpu_freq=750
arm_freq_min=900

Raising the Minimum to 900 seems to make things a little smoother. I might try raising that some more later.

I was also having some audio stuttering in a couple of games so I adjusted:
audio_pwm_mode=2
Originally it was =514
I've also heard you can try a =0

You can verify your speed with: watch -n 2 vcgencmd measure_clock arm
That shows you the speed the CPU is running at ever 2 seconds.

Overall, it's just like overclocking in Raspberry Pi OS

I have a few different Raspberry Pis, so I figured I'd turn one into a Recalbox.

I'm running it on a Pi 400.

I usually overclock my RPis, by editing the /boot/config.txt file, but that doesn't seem to be working on Recalbox. For one, it says it can't write to the file. Once I got past that, I'm not seeing an change in CPU speed with "watch -n 2 vcgencmd measure_clock arm"

Do I edit "recalbox-config.txt" instead?

What am I missing here? I've search, but haven't been able to find a good "How to" for OCing Recalbox.