I'd argue that being slow and using a ton of memory has more to do with (very) poorly optimized websites rather than blaming browsers as a whole. Sure, they'll never be as efficient as native, but people put a lot of work into optimizing browsers and most websites are never optimized.

Memory use alone is an interesting case for comparison because websites tend to have a lot more images/videos loaded than other applications. All of those PNGs and JPEGs need to be decompressed before you can paint them to the screen which makes them much larger.