Monitor display lag increasing over time

[dashsm]

Hi, I've been dealing with an issue where numerous monitors I've owned have become much less responsive over time.

Here's the timeline of monitors I've owned:
- Asus MG248 (144hz). Developed lag after one year of use.
- Another Asus monitor (couldn't find the model, 165hz). Didn't develop any lag, but developed a separate issue with vertical colored lines covering the whole panel after 2 years of use. Probably unrelated.
- BenQ EX240 (165hz). Developed lag after one year of use.

When I bought the EX240, I did a side by side comparison with the 165hz Asus, and both monitors felt identical with respect to display lag. I've since gotten rid of the Asus, but I did another comparison with a friend's 144hz Asus monitor, and my 165hz BenQ feels significantly more sluggish. I also recently replaced my old pc (1060) with a much stronger one (5070, only reused component is an 850w psu) which had no immediate effect on perceived display lag.

I've swap tested extensively with cables, outlets, ports, and peripherals, none of which had any noticeable impact on display lag. Power cycling the displays or resetting them has no effect either.

The amount of lag feels comparable to what you'd get from enabling vsync in a game; the monitor doesn't feel "broken" or unusable, and the lag is consistent across the entire panel, but it's a significant amount and it does dramatically worsen my experience, especially in games. I don't have actual testing equipment, but I recorded some slow motion videos using the flashing of an incandescent lightbulb as reference and produced a very rough estimate of 20-30 ms of time between clicking my mouse and my monitor updating. The display is refreshing about every 6 ms, so even say 15 ms of total latency is much higher than what I would expect to see. Nvidia's latency monitor gives 6-8 ms of render latency so any lag is definitely coming from my display.

My initial thought was that I just got used to the monitor so it doesn't feel snappy anymore, but it feeling "floatier" than a 144hz monitor confirms to me that something's awry. For clarification, this issue is not limited to games; just moving my mouse around on the desktop or in bios feels floaty and imprecise.

I haven't noticed any improvements or worsening across different times of day, cpu/gpu load, etc.

All the information I've found from reading this forum states that it's highly unlikely for a display to become slower over time and I don't understand how this would even be possible myself, but here we are... any thoughts? Methods for getting a more accurate measurement would also be appreciated.