Got it. I ask because the tile size will seem to directly impact the amount of data store contention (aka the number of db.TimeoutExceptions). It would be an interesting study to look at contention rates (percentage of failed datastore reads) vs. tile sizes when the app is under load.

(I also assume you make use of memcache, though I further presume that typing implies an immediate db put().)

Yeah. The tradeoff is that smaller tiles are harder on the client. I wasn't really rigorous in choosing the size, so it's likely not optimal. I actually don't use memcache yet.