The scenario where speed does cost money is when a seller panics and either underprices the home from day one or accepts the first cash offer without comparing it to what the open market would produce, both of which give away value that was never actually required to sell quickly.
A financed buyer working through the usual 2 to 4 week closing with a lawyer and Information Services Corporation title transfer is typically only a short delay behind a cash close, so choosing a slightly slower, better priced path rarely costs the seller meaningful time.
Speed and price both start from the same place, an accurate number for the specific house, not a guess about what the market will bear. Get your hand-reviewed valuation from Joel Dyck.