You want a 3B+ rather than a 3B
I’m not sure why bram has so much CPU usage with 12 MHz sample rate.
At 12 MHz it should be manageable on a RPi 3B but apparently it isn’t for his setup.
So it might work, but i can’t really be sure because for some people it seems to work, for some it doesn’t.
There are some people with many aircraft in view for whom the 3B+ is working fine with the airspy mini: Building a new receiver, this is going in a prime location - Suggestions for receiver/preamp/filter please
But it seems there are those that have problems.
A remark: you’ll need a LNA in addition to the airspy mini to make full use of it.
@bramjacobse Can you show the temperature graph and /boot/config.txt as well?
Maybe it’s throttling.
What kind of power supply are you using?