Some games are demanding. Depending on their graphics quality, they can need a lot of CPU and GPU power – set up your browser to use your graphics card. What to set

Hardware acceleration on. Chrome, Brave, Opera: Settings → System; Edge: Settings → System and performance – turn on “Use graphics acceleration when available” and restart the browser. Firefox: Settings → General → Performance – untick “Use recommended performance settings”, then tick “Use hardware acceleration when available”. Safari always uses it.
WebGL hardware accelerated. Check it under chrome://gpu (Edge: edge://gpu): “WebGL” and “WebGL2” should say “Hardware accelerated”; in Firefox under about:support → Graphics.
No energy saving while playing. Chrome's Energy Saver (Settings → Performance) and Edge's Efficiency mode (Settings → System and performance) lower the frame rate – switch them off. Laptops: plug in the charger and choose the best-performance power mode.
The fast graphics card. Laptops with two graphics chips (Windows): Settings → System → Display → Graphics → choose your browser → “High performance”, then restart the browser.
Still stuttering? Update your graphics driver, close other heavy tabs and programs, and lower the quality or resolution in the game's own settings if it offers them.