Have you used Mega.com? It does in-browser (JavaScript) encryption of uploads and downloads. And I can still hit 5 MB/s throughput. JavaScript can handle Zip compression easily.
For most users, bandwidth is in shorter supply than CPU. Especially on mobile (your constrained CPU/IO case), where people are using 3G or worse, which is often even billed by the MB.
For most users, bandwidth is in shorter supply than CPU. Especially on mobile (your constrained CPU/IO case), where people are using 3G or worse, which is often even billed by the MB.