Metadata support: Extensions handling

[Rod Widdowson]

> I'd still like to have actual numbers to back up the concern though. Most
> IdPs at scale run on multi-core systems that can handle some background
> cycles.

Yes, but there are two dimensions of scale here - number of users and
numbers of EntityIDs.  In the UK there are plenty of example of IdPs with
small user counts but which still have to process all 800 SPs.  They also
tends to be configured pretty naively.

/Rod