Yes but there's no reason why it should be struggling to work at 1Gb, PCs have been able to do that for probably 15 years.
Things to try would be to connect the PC directly to the Lila ONT and see if you still have problems, and try a clean Windows install on a spare drive, or boot a live Linux distro for testing.
Things to try would be to connect the PC directly to the Lila ONT and see if you still have problems, and try a clean Windows install on a spare drive, or boot a live Linux distro for testing.