That sounds like a hardware driver or hardware setup issue or a hardware conflict issue. Not an Adobe issue.
1) Mouse driver
2) Mouse settings
3) remove all other external USB devices, see if things improve
4) do you have a different mouse?
5) I take it this is a desktop? If laptop, remove mouse, see how touch-pad performs (if good, rethink if the mouse is bad)
6) Is this a wireless mouse? Battery level??
P.S. yes, your mouse might work just find scrolling, and moving around while looking at this web site, and go all bonkers when running a CPU and/or RAM intensive app like LR. And yes, it may behave better in PS than in LR.
I find it hard to believe that LR could somehow influence the OS or mouse driver in such a way that it wouldn't be LR's fault. Nonetheless, I tried a fresh battery in the wireless mouse and also tried a wired mouse. Neither made any difference. I also ran system monitor while doing selections and neither CPU nor memory use got above 20%. And other programs do not exhibit selection delays, even when LR is running. I've also run CPU-intensive multi-threaded programs on this computer mouse behaviour is not affected. Finally, I do experience the same behaviour on my laptop.
So, I think this falls squarely in Adobe's lap.
I tried resetting preferences, as you suggested. Big mistake. Not only is the selection problem still there, but now I have to change everything to make LR usable again.
I do not know if this would be a bug or just one of Lightroom's unconventional 'as designed' features.
I can replicate what I believe you are describing in LR 4.4, 5.7.1 & 2015.14 on Win7 and Classic 8.0 on Win10.
This is what I did to replicate it:
In library grid or filmstrip select several images.
Then move mouse pointer away from any of those selected images
Notice as you do so there is a lighter grey selection highlight that follows the pointer...
If you then right click while away from the selected images it does not deselect the initial images and the right-click operation is only performed on the image under the mouse pointer.
As it occurs in old and current versions of Lightroom I would say it is not a new performance related problem vs a design choice?
It can lead to some unexpected results if you were not aware of the behaviour.
One reason to be an 'as designed' feature is to allow you to still right-click and choose different reference photos or lock photo to 2nd monitor to compare to a selected one.
You can report the issue on the feedback forum and see if Adobe staff confirms is 'as designed' or not.
Usage of keyboard shortcuts only affects the selected items vs the one under the mouse pointer.
So this would be a best workaround until you hear from Adobe.
bhousto90: I think you're describing something different. What happens for me about 50% of the time is that I click (press and release) on an image and then immediately move the mouse. LR does not select the desired image, but instead selects the image wherever the mouse pointer happens to be ~1 second later. So, it's pretty random what actually gets selected. This cannot be construed as "as designed". It is incorrect UI implementation.