I would recommend you to disable Windows "Aero" completely and set your Visual effects to "Best performance". Also try changing in Performancne Options -> Advanced, Processor scheduling from Programs to Background services. I do not remember what my settings there are but play with it :).
Also, try to replicate the lag and while doing that, check the Performance Monitor in Task Manager - are there any CPU or GPU or HDD spikes that are unusual? This might give you some direction I think...
In Control panel, disable all AUDIO devices but Scarlett, disable all network connections, bluetooth, floppy drives, even CD/DVD drives.
I remember having similar graphic issue in the past but unfortunately I do not remember the cause or situation when I was experiencing it.