We already have heterogeneous cores these days, with E and P, and we have a ton of them as they take little space on the die relative to cache. The solution, it seems to me, is to have most cores go brrrrrr and a few that are secure.
Given that we have effectively two browser platforms (Chromium and Firefox) and two operating systems to contend with (Linux and Windows), it seems entirely tractable to get the security sensitive threads scheduled to the "S cores".