From defining the entire approach when it launched, Gmail has somehow turned into the single best argument /against/ SPAs. I dread opening it, because I know it's going to be slow to load and slow to use.
I don't know how it went so wrong. Original Gmail was a revelation. Today it's sludge.
... I just loaded Gmail on an empty cache and watched the Firefox Network pane: it made 304 requests and loaded 29.65MB of stuff (9.8MB compressed) - taking 8s to get to "loaded" (on a very fast connection) and over two minutes before it stopped loading extra pieces.
I don't know how it went so wrong. Original Gmail was a revelation. Today it's sludge.