Add to docs that trustfall is rarely the slowest part of query execution

[u9g]

Often times the slowest part of query execution is the user's adapter code, let's add that to the docs.

[obi1kenobi]

We should also add the advice about compiling Trustfall and its key dependencies for running queries in release mode, because that's the only exception to the "Trustfall isn't the slow part" here. Running Trustfall in debug mode can be a 10x slowdown, and having it compile in release mode pays for itself since one doesn't update (and therefore recompile) Trustfall every day.