That's what I believe too. Thing's are always harder to implement than you believe at first as a developer I know that. But even than the only explanation I can

That's what I believe too. Thing's are always harder to implement than you believe at first as a developer I know that. But even than the only explanation I can think of gmail's heaviness is not caring. In some ways it's even worse than a blog or news site which is often image heavy, required or not than can at least use up a lot of memory. Gmail on the other hand is not and it's something that is essentially meant to be opened for hours, gives a simple interface that suggests it would be light whereas taking up more memory than some bloated desktop client.