US-centric competitors would only support iOS and Android, whereas WhatsApp was running on the hundreds of millions of 2003-2010 vintage Nokia/Samsung featurephones that were in daily use in India, Indonesia, Nigeria, etc.
That is also part of UX. When it comes to messaging apps, Snapchat is one extreme of the spectrum -- iPhone-focused, cryptic UI, teen-oriented, majority of users in USA -- and WhatsApp is the very opposite. Both are successful examples of UX focus.
These things go hand-in-hand by the way. And that's not a complaint, it's just funny how "only teens bother to figure this out" can be an honest-to-goodness feature under specific circumstances. Especially if you (basically) need to be "introduced" to a program and all of its quirks through your friends.
It is already blazing fast with only HTML and CSS, as the Web was designed to be.
EDIT: typo
Only mobile, the web version is shitty because it hosts a web server on your phone which is unreliable. And even with that you can only access the web version from one computer and it does not work well on tablets.
I always come back to the example of WhatsApp and how it took on SMS, IM, and emails and won. People claiming WhatsApp went in mobile first are getting it only partially right. WhatsApp defined itself as a replacement for SMS: No login passwords, no chat history, cross platform and that led to them being cheaper, better, faster, frictionless. Later added MMS-esque capabilities like media and then email-esque features like Groups with super nice execution on the UX front.
Similar thing for AMP pages: It just works and it is blazing fast.