Question 144hz monitor feels like it's not 144 even when gaming at 144fps+

Jun 21, 2019
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I got the Asus VG248QE 1 month ago and it was fine until one day i started to notice some kind of lag spikes and when the days went past it got even worse it feels like im playing on 100 or 120 hz monitor (PC specs : Gtx 1070ti gainward , i7-8700 , 16ram ddr3 2666mhz)
Main game I play is Fortnite


P.S does anybody know if it's monitor problem or hardware/os problem