After a stick of RAM failed in my laptop, this is the information I wanted to find but couldn't. I replaced it with a smaller stick, putting the system into Intel's 'Flex' memory mode. Performance in things such as video encoding and gaming were unaffected, but the entire system developed a strange lag, e.g:
- Firefox dropped frames like crazy, scrolling reduced to 30-40 fps where it had previously been solid 60
- Explorer lagged while scrolling folders
- Various animations throughout Windows dropped frames
Even more oddly, this lag was intermittent/random - it would be consistent while scrolling, then I could swap to another app and do something else, then I could come back and it would be perfect - and vice-versa.
I swapped the other stick to form a matching pair, and the lag completely vanished. It seems that proper dual channel is essential for laptops with high DPI displays.