I believe that the problem has now been solved. In comment #9 I remarked that adding i915.enable_psr=0 to the GRUB_CMDLINE_LINUX_DEFAULT string didn't appear to work; but in fact it does. The reason for the discrepancy was that I have a dual-boot system with two copies of Lubuntu, and two copies of GRUB; and I had made the change to the copy of GRUB which wasn't being used. Now that I have made the change to the correct copy of GRUB the problem has gone away. I also notice that scrolling is much smoother.