From memory when I worked there 10 years ago, all the Javascript based stuff was pretty slow in a very noticeable way. They've likely gotten better as they are using a well known Javascript and rendering engine, so they're getting many optimizations for free.
They're very involved in web standards. Rob Palmer from Bloomberg is co-chair of TC39.
We’re in the middle of TC39 plenary in Tokyo right now as a matter of fact! Rob regularly posts on X as things advance through the staging process, and today is the final day.