I recently acquired a brand new RPi Model 3 B+ and decided to try and install Retropie (v4.4) on it. I managed it to boot properly and I got a PS3 controller to work via USB.
The problem is that I continuously see the under voltage warning and sometimes the system lags down a little and I suspect this is the cause. I'm currently feeding the RPi with 5V, 2A. Are the amps the problem?