I had same issue with my dell inspiron 5577 gaming before, but it was due to heating issue. Dell changed thermal paste and it resolved now.

As per my knowledge the cpu might be throttling due to the apps running in the background. I know thats and i7 8th gen processor, but can you please check if it's due to cpu reaching 100% in task manager. Also check the memory usage.

Hey thank you for your answer, currently I`m checking the task administrator and CPU is between 25 and 50% Memory is 91% which I think is a lot because the first 2 months I had this Laptop never had a problem with that, actually I was able to have 2 or 3 games open at the same time, playing one of them and the other in background, and now I cannot even open Opera, chrome or any other browser and a single game at the same time without the FPS Drop,

Currently CPU 25 - 40%, Memory 84%, GPU 30%,

Download MSI Afterburner with RivaTuner statistics server and setup On Screen Display to show GPU and CPU(especially CPU temps) on the Overlay while gaming.  Play a game with the OnScreen Overlay for 10-15minutes and monitor your CPU temperatures and core clock speed.  Its probably Thermal Throttling.  I had the Dell G3 for 6 months,  at first I never really had any issues(but it was mostly during the winter time).  As soon as spring hit I started to notice it stuttering in games witch prompted me to start monitoring things in game.  As soon as I started watching the CPU temps in afterburner I found out I was hitting 100C constantly and throttling down to 2ghz core clock witch kept going on an endless cycle.  I repasted the CPU/GPU heatsink with Thermal Grizzly cpu paste and that helped good deal,  but it still took using ThrottleStop to undervolt my CPU to drop temps to a respectable level.  Mine was running an I5 8300h and I kept my heatsink and fans very clean of dust by taking the laptop apart and spraying out the fans and heatsink with an air compressor pretty regularly.

Also try watching your HDD activity in Task manager,  when I first got my G3 I had nothing but a Seagate Firecuda 5400rpm SSHD installed for storage.  The first thing I noticed was while gaming I would get stutters and in task manager I would see my HDD activity being maxed out by windows Superfetch(sysmain) and this caused my games to lag like crazy.  After disabling the Superfetch service it completely iradicated that problem.

I also believe that running the CPU at 100c constantly may have caused damage to my laptops internal heatsink so since I was still under warranty I contacted dell support for a replacement heatsink/fan assembly.  After being on backorder for 2 months dell contacted me to just do a system exchange and replaced my G3 with a G5.  Thermal problems are completely gone now and everything functions so much better.  The G5 has a lot better cooling.  I think to G3 design may just have a horrible time keeping things cool(the screen also blocks the rear exhaust ports.

Anyways if your still under warranty try contacting dell if you are experiencing really bad thermal issues.

Sound like this and mine problem. Did you check your battery state at lag? Mine laptop goes to battery mode only for a second,while it plugged in and it made lagging,becouse on battery mode it has only 2.3Mhz frequency,while at AC about 3.9.