it means that it will not be filled fast
when the system reaches pressure.



Let me revise my comment a little.

You're a math guy. How will the big tank _ever_ fill quickly, and still
not prevent you from getting full pressure quickly in the small tank?

Opening a 'pass-over' valve to the big tank once the small one is full
won't do it! It'll drop the system down to system peak divided by the
ratio of tank volumes the INSTANT you open it (well, within seconds). So
the pump will come back on, and now run an intolerable length of time
before you have enough pressure to work. You can't have 'quick' and
'full pressure' in the same formula.

Pump capacity is the limit; always.
Lloyd