It's not clear to me what you might be referring to as far as video lag. Not responding to mouse input as quickly as you're used to or ?
As far as LatencyMon, I wouldn't be too worried if you're no longer getting the audio pops and dropouts. I recently installed a Windows 10 Insider build and ran a trace of my system. It was showing quite high DPC processing times for DPCs running on cores other than 0 and sure enough it was associated with ntoskrnl.exe. One took as long as 17 milliseconds which would normally be considered terrible and yet my system was running better than it has since the Spectre/Meltdown updates last year. So I think Microsoft have done some tweaks regarding ISR/DPC scheduling to make better use of multiple processors in the Insider builds and I doubt LatencyMon is taking those changes into account yet - if ever they do. When I reverted back to build 1809 all my DPCs went back to sub-millisecond levels so if you're still using an Insider build it might be worth trying the 1809 build to see if your DPCs also become more reasonable.