Gmail uses stupid amounts of memory, and the web version on iOS is so terrible it's got to be deliberate. The key problem is that they override scroll behavior such that scrolling intents are often registered as clicks, then they reset scroll position on back, the combo of which makes it almost unusable if what you're doing involves scrolling your mail list at all.

They used to still offer "basic HTML" gmail, which was waaaaaay better all around and was the only way I used it on any platform, but they discontinued that some time back.