You need much much higher doses for it to work orally.
Think about it: the biological half time for salvinorin is extremely short (15-20min iirc), so if you're looking for onset slower than that, you basically have to be continuously overdosing for it to build up in your system (you need to be consuming it at rate higher than it gets eliminated)
I know this is how it's been traditionally consumed, yes, but those are intense group sessions you probably can't reproduce as a relative noob.
So, tl;dr: it's not the best choice if you want slow onset. It's for getting hit hard fast.