Are per-user tags efficient / okay to use?

[Andrew Sutherland]

Q's regarding the private per-user per-bug tag mechanism (not the
publicly visible comment tag mechanism):
* Is it going to stick around for BMO for a long time or is it something
on the chopping block?
* Is it efficient enough that it won't make the server angry/slow:
** For simple tags like "important", "boring"?
** For tupled-tags storing a key and a value, like
"importance=highest"? (I'm speculating that the field gets normalized
so every tag gets its own integer id and the answer is that this is
probably bad but not as bad as using the tags like a private whiteboard
field.)

Context: I'm hobby hacking on a mail reader derived from the remains of
the FxOS mail project and want to improve my bugmail experience. I'm
tentatively planning to add a bugzilla account type and a design
question for me is where private metadata should be stored on servers so
that it can be replicated between devices. (And so that if my project
fizzles the metadata isn't trapped.)

My gut assumption is that BMO doesn't need people storing more data in
it and the best option is just to hybridize the account so it stores
private data in IMAP/JMAP using existing tag/label systems and ANNOTATE
when fancier stuff is needed. This nicely exposes things to other mail
clients, etc. But I thought I'd check, especially if any of the plans
for built-in fancier bugzilla UIs would use tags or some other upcoming
mechanism that lines up with my use-case.

Thanks!
Andrew