The gift of Spanner is it’s always slow, so if you start there you will naturally gain experience in either hiding latency or relaxing consistency requirements, which are both good skills.
Spanner is pretty fast, but as with any database you have to use it correctly to get the speed. Since it’s a non-standard database, it requires non-standard tricks and skills.
No, that's about right. You get one write per second per entity or something like that and that's as fast as it goes.
The key is to choose your entity wiseley. I wrote an app inside Google that did on the order of 10,000 writes per second and it was no problem; each entity group only showed up once every minute or so.
AFAIR, entity group is a Megastore concept. Megastore is ill-conceived predecessor of Spanner, are you confusing them? Spanner is much faster than 1 transaction per tablet.
