Game lagging out of nowhere

@stickysara  Yes, a processor will thermal throttle in the 90s (Celsius) and can therefore slow down any and all programs currently running.  If you're comfortable taking off the bottom panel of your laptop, it's worth doing so and cleaning the vents and fan, making sure the fan can rotate properly as well.  This doesn't involve taking apart any hardware; you'd only nneed to remove some screws and the panel.

If you're not comfortable doing this, a computer repair shop could do it for you, for a fee, but in that case it's worth blowing compressed air into the vents to see whether it helps.  A can of compressed air is much cheaper than an appointment with a computer tech, if also somewhat less effective.

Additionally, make sure the laptop is always resting on a hard flat surface while you play; no running Sims in bed with the laptop on the covers.  If that's not enough, you might consider a cooling pad or tray, but that won't fix a major problem with cooling such as a blocked vent; it can only help around the margins.

Thank you for responding once again. I actually checked my fans and they didn’t look particularly dusty but I am still going to take it to a repair shop so they can clean them. I’m also curious if worn down thermal paste could be an issue? I’ve had my laptop for 5 years so I’m not sure if the thermal paste has worn down or not. If it has, could it be the cause for the performance issues as well?

@stickysara  Thermal paste doesn't really wear down, but it can dry out and therefore become less effective.  Reapplying it is something you might not want to do for the first time on a laptop, especially since that involves removing the heatsink and perhaps some other components, which in turn can be complicated or delicate depending on the laptop's layout.  It's certainly a possibility though, and one that should be considered if other interventions don't help.

So would taking it to a repair shop so they could replace the thermal paste be worth it? Could the dried out thermal paste be causing the throttling? My fan isn't dusty enough to be causing the issue, and I had them cleaned recently as well. 

@stickysara  The answer is... maybe.  I would be mildly surprised, but not shocked, if your laptop's thermal paste were so dried out that it absolutely needed to be replaced.  Laptop manufacturers know that their products could be used for a number of years and generally prepare for that possibility, and Asus generally makes quality products.  It's much more likely that dust clogs a fan or a vent, or something else prevents the fan from spinning properly, before the thermal paste itself fails.  On the other hand, it's far from impossible—it would happen eventually, and maybe your laptop's thermal paste just dried out ahead of schedule.

The other consideration is whether you want to invest this kind of money in your laptop.  A professional cleaning is usually relatively cheap, but having someone take apart the laptop and apply new thermal paste probably wouldn't be.  It would still likely be cheaper than a new laptop, but on the other hand, a decent new laptop would be significantly faster, and if you're going to get one in the near future anyway, you might not want to spend so much on this one.  This is where I can't tell you the right answer because I don't know your budget or what your local shop would charge, so think about what you want, and call around to get some estimates.

And of course you can't know for sure that the thermal paste is the issue until someone checks.  I'd still try a cooling pad or fan, and borrow one from a friend to test if you can, to see whether you can keep the processor below 95º C while playing.  A max in the low 90s is fine and shouldn't trigger thermal throttling.  You can also upload an hwinfo log for me to look at if you want.