FYI you can still switch to the old design. When the fake loading screen (https://smitop.com/post/gmail-s-fake-loading-indicator/) is loading, click on the bottom on "Prefer old design". Then you can set that as default. It is an absolute must for me given how fast it is.
Note that objectively speaking the basic HTML gmail is slower in every aspect, except for the initial page load. Everything but the initial page load is slower. The main web app has extensive latency-hiding features that are key to Gmail's overall performance story. The basic HTML UI has none of these features. Every action in the basic UI takes half a second or more because it's a server round trip.
Example: load the basic HTML and enter a thread then return to the thread list. Returning to the thread list takes about 1 second. In the full gmail return to thread list takes no remote time, only local event handler CPU time, it's almost instantaneous.
Example 2: Open a thread in the basic UI and use "newer" or "older" to navigate to other threads. Every click takes ~1s. In the full Gmail UI navigate to adjacent thread takes almost no time, because the app pre-loads these threads in anticipation of you visiting them.
TL;DR the basic HTML gmail is good if your device is severely resource-poor and you just need to see if you have mail. For every other purpose it's worse.
