> Unless there is a usecase for averaging gazillion numbers on the client

Adding 2 numbers (x4) is just the simplest thing you can do.

There are a bunch of other operators and methods:

https://api.dartlang.org/apidocs/channels/stable/dartdoc-vie...

Good intro:

https://www.dartlang.org/articles/simd/

GMMs in speech recognition:

http://dartogreniyorum.blogspot.com/2013/05/performance-opti...

(Be sure to check the follow up article for the updated numbers.)