Probably wifi, I dont think Moonlight Embedded uses much ram. I also get the undervoltage warning nearly constantly, since the a+ is powered by the usb port of a projector. Maybe that also affects things.

To add more details, I use Sunshine as the server, and stream 1080p, in HEVC for the pi 4 and 5, and h264 for the 3 A+.

I use Moonlight Qt on a raspberry pi 5, and used it on a raspberry pi 4 before that. Both connected via ethernet, streaming at 150 mbps. It works very well, feels like being at the computer. It feels like there is next to no delay, and moonlight reports around 5 ms.

Somewhere else I use a raspberry pi 3 A+ with Moonlight Embedded, connected via Wi-Fi, and it works pretty well, but I can notice the delay a bit more. Still able to stream at 40 mbps.