Mouse lagging on iMac
I was having similar trouble with a Logitech bluetooth keyboard and mouse, when I plugged the transponder dongle into the USB port on the back of my iMac.  Researching the issue, I found several people suggesting to move the USB dongle away from any USB 3.0 ports, and as an EE I figured it also wouldn't hurt to get it closer to the devices it's talking to.  So this is my setup now and I've been a happy camper ever since I bought a USB 2.0 hub, plugged the dongle into that, and moved it near my keyboard and mouse.  (Of course the hub is connected to the iMac in the back -- should be obvious, but just sayin.)