XL2411P feels slow,unresponsive and shows vertical lines on the monitor

[Chief Blur Buster]

This sounds like a glitch/defect but it also happens with overheating conditions.

You may wish to do all:
1. Canned air dusting of monitor vents;
2. factory reset the monitor;
3. Use Instant Mode
4. Cold boot the monitor from power-unplugged state
5. Rung with extra cooling from now on
(Run a desk fan to the side, sidways airflow, cooling both rear/front of monitor)

Overheating can cause some scaler/TCONs to suddenly do some very weird glitches (including picture quality and lag quality). Something in the monitor code crashes, throws an exception, and the firmware watchdog resumes processing in a less-efficient mode or limbo mode (random register settings that is bad for picture and lag), etc. Monitor settings flash memory becomes corrupt too, even for hidden settings (e.g. even settings not visible in factory menu). Running in degraded operation after a crash (in monitor electronics) is classically considered better than a screen going blank during a monitor scaler crash, but very annoying for esports.

It's summertime in the upper half of this spinning globe, temps are hot, and monitors can glitch in overheat conditions.

But it might also just be a defect that creates random scaler crashes that triggers a resumed monitor operation in degraded performance mode. Replacing monitor under warranty may get you a fixed specimen or at least a more thermally-resistant specimen.

Bear in mind the monitor processing is literally a (simple) computer unto itself. Esports monitors have more powerful processing than simple 1080p 60Hz screens and lesser, and may be pushed to a heat envelope that makes it unusable in hot rooms (Many monitors specify operating temps under 30C in their specs). The "computer" in your esports monitor is more powerful than the Cray supercomputer of 1980s, and it is prone to all kinds of tech glitches that befalls computers. If your room is room temperature and doesn't have any high room temps (e.g. non-air-conditioned room in mid summer), then this would typically be an RMA-able glitch if not traced to an obvious out-of-spec operation like running beyond temp range specified in the manual.