I think "tempo loss" can be broken down into two parts: one is, you can start drawing large chunks of your deck even with light trashing and getting the trasher is a turn spent not getting more draw. Another is where trashing interferes with your economy in a way that detracts from building. That is, the decrease in draw is not (only) from not buying draw on the turn where you buy the trasher, but (also and primarily) from not buying draw on later turns.
I've recently played on this kingdom:
(Estates and Provinces, no landscapes)
I want to draw my deck with Festival and Library, to do it I want to trash, I want to be very treasure-light and I would like to get an Artisan to help me get Festival and Library if I can find time to do it. How should I open? Thinning is winning so I get Silver and Moneylender, and my first $5 is Sentry, yes? Or maybe even Moneylender/Cellar to play Moneylender more often?
Maybe, it looks plausible but I'm not super duper certain: opening Silver plus terminal silver gets your money to $11 in 12 cards or slightly less than $1/card (= $5/hand on average). Moneylender only trashes Copper and Sentry only increases your money density on average if you either selectively trash estates or your money density is already at least $1/card or you look at Copper+Estate. How do you hit the next $5? A second silver is unattractive in your eventual Festival/Library deck, Militia fights with Moneylender for terminal space (as does Vassal early), so Poacher? A fine card that you're happy to have, but are you happy with that plan for hitting $5?
Getting a Festival as your first $5 helps your economy while also making it more attractive to add a second terminal silver. You might want to delay trashing until your economy is at the point where trashing actively improves your economy (i.e., on this board, your chance of hitting $5). So maybe Moneylender/Silver, Festival as your first $5 and Sentry as the second $5, then more Festivals?
... or did I just get burned by a bad experience that was due more to variance and bad luck than the fundamentals I'm trying to point to, and I'm rationalizing it post-hoc? I had a hard time hitting $5 opening Moneylender/Silver.
In any case, I think the general pattern I'm talking about is clear: missing a key price point really hurts and the particular trashing on the board hurts your chances of hitting that price point until you build for a little while, yet you also want to trash. (And the trashing is on the slow side, maybe.) On those boards I think it makes sense to delay trashing. Identifying those boards seems non-obvious. Perhaps non-obvious enough that I failed