> "Plain HTML" gmail vs. any other version. It's way faster to navigate.

And offers far less features. Attaching a file to an email takes ages without drag and drop, this alone is worth waiting few more milliseconds to most people (Gmail loads in less than 2 seconds on my crappy hotel wifi)

It is not milliseconds on older devices and network is not always the bottleneck

Most of the time you read e-mails, not write them.

When you write e-mails, most of the time you don't need multiple attachments. The first attachment is reasonably fast to add.