Game engines and browsers have radically different architectures and performance/productivity requirements. Computing polygons is basically applying one function to an array of values and is delegated to the GPU, so its no wonder they easily reach 60 FPS with hundred of thousands of polygons per frame.
A better comparison would be against the performance of game logic, which in a game engine can easily become the bottleneck from all the conditionals, random memory accesses and sequential dependencies. And even there they don't sit on nearly as many layers of abstractions as a web page does.
This comes at the cost of productivity; building a web site is a few orders of magnitude more productive than using the latest game engines. Of course, sitting on the DOM and its many quirks as well as its reflow/redraw cycles can make it harder to reach 60FPS in the browser.
But at least you're saving yourself from a world of low-level pain.
A better comparison would be against the performance of game logic, which in a game engine can easily become the bottleneck from all the conditionals, random memory accesses and sequential dependencies. And even there they don't sit on nearly as many layers of abstractions as a web page does.
This comes at the cost of productivity; building a web site is a few orders of magnitude more productive than using the latest game engines. Of course, sitting on the DOM and its many quirks as well as its reflow/redraw cycles can make it harder to reach 60FPS in the browser.
But at least you're saving yourself from a world of low-level pain.