I was having awful SL lag after a large Windows 10 update even though I have a powerful gaming computer and a extremely fast FIOS connection. I rolled back to the previous installation of Windows 10, and that seems to have removed the lag. I am so surprised that I can't find any official comments from SL about this, since I have heard about this problem from others. Whether it is an SL issue or not, SL should investigate it and come up with a workaround. The lag was so severe that SL was unplayable.