An old laptop is not slow at everything. It is slow at three specific things, and almost nothing a browser game does is one of them. Once you know which three, the question stops being "will this run" and becomes "which of these is worth the evening", which is a much better question.
What actually makes a machine struggle
Three bottlenecks, in the order they bite. Memory is first: four gigabytes with a browser open is already tight, and every extra tab is a bite out of it. The graphics chip is second, and it only matters for games drawing a real three-dimensional scene. A map, an interface and some sprites do not trouble it. The disk is third, and it is the reason an installed game takes two minutes to open on a machine where a browser game takes four seconds.
Notice what is not on that list: the processor. Nothing in this category is doing enough arithmetic to bother a chip from the last decade. If your laptop plays video in a tab without stuttering, the hardware is not what stands between you and a game.
The browser is half the problem
A game that runs badly on an old machine is usually a game running badly in a browser with forty tabs open. Three habits fix most of it, and none of them cost anything.
- Close the other tabs, or move the game to its own window. Memory is the bottleneck and tabs are what eat it.
- Turn on hardware acceleration if it is off, and turn it off if the game flickers. One of the two is right for your machine and it takes thirty seconds to find out which.
- Leave the extensions that block things running. A page that loads fewer scripts is a page an old machine renders faster, and it is the single biggest difference on a slow connection.
If the fan is still audible, the game is drawing more than it needs to. Almost every game in this category has a graphics setting buried two menus deep, and turning the particle effects off costs nothing anybody plays for.
What the first twenty minutes should tell you
There are two kinds of browser game, and you can tell them apart quickly. One hands you something to lose. The other hands you a menu and a currency you have not earned yet. On an old machine that difference matters twice over, because the second kind also tends to be the kind that loads a video advertisement between screens.
- Something built, owned or lost inside ten minutes.
- No launcher, no patcher, no forty-second logo sequence.
- A reason to open the tab again tomorrow that is not a daily reward.
What to look for in a game itself
Two things decide whether a browser game is comfortable on modest hardware, and neither is the genre. The first is whether the scene is flat. An isometric city, a card table or a two-dimensional map is drawing far less than a rotating three-dimensional world, and the difference is the whole gap between a quiet fan and a loud one.
The second is whether anything happens in real time. In a turn-based fight or a game where travel is measured in hours, a dropped frame costs you nothing at all. In anything with a reflex in it, the same dropped frame is the difference between winning and not, which is why older machines feel worse at those games than the numbers suggest they should.
Battery, heat and the thing nobody mentions
A game that runs at three hundred frames a second on a laptop is a game turning your battery into noise for no benefit. If the browser offers a frame limit, use it. If the game offers one, use that instead. Half the complaints about an old machine being "too slow for games" are actually complaints about it being too hot to sit near, and a cap fixes that in one click.
The other unglamorous fix is the surface the laptop is on. Vents on the underside plus a duvet is the most common performance problem in this entire article, and it is free to solve.
When it really is the machine
If the browser itself stutters while scrolling a plain page, no setting in a game will help. That is memory, and the honest answers are fewer tabs, a lighter browser, or more memory if the machine takes it. Everything short of that, in this category, is solvable in the settings you already have.
The short version
Close the other tabs, cap the frame rate, get the vents off the sofa, and pick something flat and turn-based. On a machine that struggles with everything else, that combination is usually the difference between a game that runs and one that does not.