In system freighter class Freighter 4950 tons 210 Crew 399 BP Signature 99-315
3181 km/s Armour 1 Shields 0-0 Sensors 0/0/0/0 Damage Control 0-0
Cargo 25000 Tractor 20 Supply 200
Ion Engine IB-5 (5) Armour 0 Exp 8%
Has a loading time of 12 hours and 30 minutes.
This is an example of what I consider to be a good starting freighter. For short hauls within a single system, speed isn't as critical as being able to load and unload quickly.
The asteroid belt is only a couple days out. A Themistocles II would get there slightly faster, but while it would take, say, 5 days travel instead of 6, it would spend 4 days loading and unloading instead of 1.
It is competitive with the Themistocles design for round trips whose travel time is 15 days or less for the Themistocles.
With the Asteroid resource bug fixed, there will be a lot of system where one asteroid after another will be mined. Having an in-system freighter shuttling automated mines about would be more efficient than using long haul freighters like the Themistocles.
For round trip distances of 6 days, an In-system freighter could shift approximately 50 mines a year, while the faster Themistocles could only shift 40.