Well, there goes that theory.
I'm supporting your theory. I'm on my second shield k1 tablet. Returned the first one because it seemed to have touchscreen issues... Swiping in chrome would cause the page to scroll a couple lines in the correct direction, and then 50 lines in the opposite direction. Same thing would happen with the swipes just on the homescreen. Playing games such as Marvel Contest of Champions was nearly impossible. Taps would register as swipes, holds would register as taps, swipes would register as holds. Didn't happen ALL the time, but guaranteed to happen every single session for at least 50% of the time.
Second tablet had the same issue. Tried screen recalibration. Tried wiping cache. Tried accessibility options off. Tried changing performance between max, optimized, and custom.
It finally dawned on me that I didn't have this issue on my old OG shield tablet, and didn't start having the issue (that I remember) on the k1 until after the marshmallow update. First thing I did was after the update was excitedly set my sd card to adopted internal storage... these shield tablets and their pitiful 16gb of internal storage, ugh!
Anyway, this morning I got the idea to set the sd card back to portable storage. The tablet is brand new as of yesterday, so nothing to lose by reformatting the sd card. Format was done withing a minute. Rebooted the tablet for good measure
Low and behold, no lag or touchscreen issues so far!
I'm going to extensively test it today. Will report back with any changes or updates later! Hopefully a few more of you guys are willing to try this out as well. Might just fix your lag and touch issues.