I wouldn't mind a small delay. Say about 10 seconds + 0.5 seconds per tonne.
This would be for both loading and unloading. The unloading starts as soon as you start selling, the loading starts when you start buying. These two separate animations can overlap.
Doing this would result in a fully loaded Cobra being reloaded in just over 27.5 seconds (35T), a Sidewinder in 12 seconds (4T), and an Anaconda in 124 seconds (228T).
Express options (at cost) can speed up reloading of large consignments, (say) in batches of 40 units, at 10 seconds + 0.125 seconds per tonne (full-loaded Anaconda - 38.5 seconds). Maybe the fee could be ~3% of the cargo value?
I don't know what ships will have what capacities, so my figures are just a guess, but I think doing something along the lines of the above would scale the trading option well.
Hopefully a player with a large cargo-hauler would feel like they're really dealing in bulk quantities of goods, without the increased reload-time becoming a drag, and it being merely accepted as just the way things are when trading.