Firebase currently doesn't support regions. My understanding is that it is based in the US central Google Cloud region.
If I am serving an app in London, which in turn calls a google cloud function, which in turn calls Google App Engine, which in turn calls a database, then I'm under the impression that this would involve circuitous routing.
For example:
1) Web Application served in London (CDN) ->
2) Firebase database (US Central) ->
3) Google App Engine (US Central) ->
4) Database request (London, UK) ->
5) written back to Firebase database (US Central) ->
6) served back to Web Application (London, UK).
With ~30,000km of cabling to travel through, would the general impact on performance / lag be sufficient to significantly affect app performance?