Input lag/delay while gaming, de-sync issues in some games - Xbox and PC

I had Xfinity service activated at my home a couple of months ago. Aside from one hiccup with power levels being out of spec for my modem, I haven't had any other issues. However, for about a month now, I get a considerable amount of input lag in some games like FIFA and Madden, where this wasn't an issue before. Every action takes what seems like half a second to register, and gameplay feels very sluggish. When I play CoD or CSGO, I have issues where what I am doing on my end clearly isn't syncing up to what the server is receiving from my end. I'll get hit markers on an enemy and die, even though I fired first, or clearly get the drop on someone in CSGO and die without them actually firing a shot on my screen.

I've swapped out my router and modem. I don't show any errors or power levels out of spec on my modem. I also hooked up my PC and Xbox to the modem directly and didn't notice a difference. I had one day where everything played as it should, on a Tuesday last week. But the few weeks before that, and the days since, it's the same sluggish experience on sports games, and odd de-sync issues on FPS games. My ping is low, I have no packet loss, and jitter is also miniscule. I asked EA reps for their games what the issue could be, and if it's regarding servers on their end, and they told me to contact my ISP since they couldn't find a problem on their end.

I understand that gaming traffic isn't prioritized, but I have actually stopped gaming the past couple of days because the experience isn't enjoyable at all. I would appreciate some clarity on the issue.