3. For good cooling, you need
- high thermal conductivity
- low thermal mass
- as much surface as you can
A big block of aluminum doesn't tick one of these.
Thermal mass is not a bad thing. It acts like a heat sink to store the excess heat and take it away from the circuit board.
What you want is low thermal lag, but in general items with larger thermal mass also have higher thermal lag. You can remedy this with lots and lots of surface area (fins)
So a big block of metal designed well (lots of cooling fins) would help. If it didn’t, Noctua wouldn’t be successful selling cooling fins/fans for computer CPUs