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They both operate on 2.4ghz. It seems like when you move them away from the back of the computer that it gets better. Also I presumed that they are rechargeable, is that accurate? If not have you tried different batteries including a different brand?
Perhaps the main board wasn’t properly or sufficiently grounded the or the I/O shield didn’t get installed properly or not at all? The IO shield is the thin metal piece that came with your main board that gets put into the opening in the back of your case.
Do you have a multimeter? If so can you check the resistance of the ground wire in your power cord? Make sure it’s unplugged at both ends... checking resistance from a live wire to ground probably wouldn’t yield very good results. If not you could go buy another one to test out. Or swap it from your monitor.
If all of that checks out then without some equipment to do frequency scanning I’d go out on a limb and say it’s remotely possible that your power supply is causing the interference.
Without figuring out what is causing the interference you’ll want a USB Hub to put near the keyboard and mouse. Please get a shielded cable to go from the computer to the hub. You could also consider getting a couple of something like the link below however I’m not sure if you can get them with shielded cables.
You could get a dongle with a high gain antenna and a 5ft extension cable with a stand ([link]).
That would also give you a way to determine if it is interference: connect the dongle to the front port using the extension cable, and confirm it works, then without disconnecting anything, physically move the dongle behind the case and see if the lag appears.
Also FWIW, I really like this extender cable: the end is a nice solid base that sits on your desk. But the one linked above will work just as well.
I recommend the StarTech desktop extension cable. Much better than loose cable.
Or a desk grommet with usb ports if you can drill a hole in your desk.