Seconding the default. The threshold where a dedicated store starts earning its keep is much higher than the marketing implies, and below it you are running a second system for no measurable gain.
The specific thing to watch on the general-purpose route is the index type and its parameters, because the defaults are tuned for small data and the falloff past a certain row count is steep. That is a tuning problem, not a reason to migrate, and people migrate instead of tuning.