The tags plugin is excellent, however, there are some pretty big performance issues.
One is, regardless of specifying a realm (via tags search page or a macro), all tag providers are asked for their tags, and only after that is the realm checked.
The second is checking permissions on each ticket (in the ticket provider). This is fine if you have a few hundred (or thousand) tickets. In a database with a couple hundred thousand tickets, the tags plugin becomes unusable. Relying on a ticket_view and tags_view at a "global" level is sufficient for the majority of cases.
The attached patch addresses both issues above, though I don't regard it as a "final" patch.