It is obvious that making up a protocol to notify all the instances is a non-starter. Sharing a read-only memory page that has nothing but the pixels in it is trivial, decoupled, and provably not a security hole. Probably there are already such pages in use that the pixels could be added to. Any Terminal that has customized its seconds display would not be obliged to participate. And, there is no need to interrupt a busy core to update the seconds display; the update can wait until it is not busy.

In any case, they could each render the ten digits, or even 61 digit pairs, ahead of time, and blit them into place as needed.

But what keeps Windows users from having nice things is that they all, without exception, depend on the whims of those MS engineers (not to say hacks) who get to decide, "Yeah, nah" on their behalf.