A little bit of everything. I think the biggest drawbacks of even the latest Raspberry Pi are:
- USB bandwidth shared between USB and Ethernet.
- Raspbian keeps binary compatibility with the first generation Raspberry Pis so that the software does not take proper advantage of the CPU features. So sure, you get 4 cores and more MHz but there are useful instructions that are not being taken advantage of.
The last point above is why I started with building my own compiler then build all critical gqrx dependencies from source. Eventually, this lead to gqrx running on the RPI2 and RPI3 whereas the gnuradio and gqrx packages distributed with Raspbian are quite useless.
All that said I’d like to point out that I think they did a really good job with the Rapsberry Pi 3 and Raspbian OS. It’s just that gqrx comes with a lot of bloat and difficult to distribute in a target specific binary form.