simple

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It depends, a lot of modern AAA games run fine but a good amount of them are very unoptimized and run poorly on the Deck.

Valve said not to expect a hardware refresh soon since newer chips are more expensive and not that much faster, just look at the handheld PCs selling for ~$900 that are only 25% faster.

There are plenty of benchmarks online so you can check if the games you want will work properly or not.