It's unlikely this will get better. From what I can see, Proton does elliptic curve crypto in pure JavaScript (and may even switch to WebAssembly for that). I bet it's simply too slow and the Promises time out before this is processed - assuming it'd work in first place, since that stuff is screaming endianness issues to me. Unfortunately, the recent advancements in JavaScript were done by people ignoring the existence of Big Endian.