Start with the browser, not the game
Confirm hardware acceleration is enabled, keep the browser current, and play in a window with as few other tabs open as possible. Extensions that inject scripts into every page are a common and easily-missed cause of stutter.
A separate browser profile used only for games keeps extensions and cached data out of each other's way.
Then tune the game
Lower particle and shadow settings first — they cost the most and change the look the least. Cap the frame rate to your monitor's refresh rate rather than letting it run free.
If a game stutters only after a long session, it is usually memory pressure. Reloading the tab every hour or two is an unglamorous but effective fix.