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Can't second this. Had the Standard Wifi for about two weeks (Tokyo, Kyoto, Osaka, Hiroshima and some kilometers around the cities) and the speed was incredible slow. Using Google Maps was ok, but surfing (i.e. tripadvisor, /r/JapanTravel) or sending/receiving pictures (Email, WhatsApp) was slow^slow^slow. The 2nd problem was, that the battery just survived 4-5h of continuous work (and not 8-10h).
Maybe it is just this provider, maybe all providers have this issues. Don't know.
On the 2nd or 3rd day I've bought an offline app and marked the spots in the hotel (or Starbucks...). And then I've used this app for offline navigation. [link]
> Map data and POI are provided by OpenStreetMap
Seems that one is too. Like you though I use OSMAnd.
I've also done few thousand changes/additions to my city to improve the map. I encourage everyone to do this for places they are familiar with to do the same, so it's a better experience for everyone. It's surprisingly fun too.