In a situation like that if I understand it correctly you've got a tradeoff between default storage and plaintext storage -- size vs speed. You also have aggregation options that, without knowing more details, I would imagine would resolve this. This sounds like an infrastructure architecture problem and not necessarily a journald problem from what I've read here. Interesting stuff.
I wouldn't recommend in any distribution to use the default logging configuration for scale.
Configuration management and some planning are due there. Sure, it's slow. I can't blame journald for that, I would expect it to be slow.
Re: breaking everybody's setup by moving legacy logging tools to a secondary repo-- I would imagine a transition window would be needed. Suse dropped lilo eventually. I'm sure out there, somewhere under various rocks and lurching in untold caves are people still mad about it because their setup worked.
If it gets too much pushback we could always just call it DevLogOps or something and let the problem solve itself.
-C