That seems pretty reasonable. That's for the app-server cluster to generate uncached results, and propogate them. In general the app server will do this upon the change occurring, before a user actually asks for the page/information.
Users should only really see this when performing "mutable" operations (submitting edits, adding new pages or content) or when searching uncommon queries.
I doubt it's anywhere close to the critical path for the average guest, casual user, or even contributor. I'd suspect the only type of user who would find themselves hitting those appserver requests frequently would be moderators and admins.
Also: 500 ms on wikimedia sites is very much still in the "okay" range, subjectively. They aren't really sites that you make requests to every minute, if loading the next article took 500ms every time then so be it.
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