I have put my hands on a Raspberry PI 2, and I have found that it takes 6 minutes to compile the backend code using the 4 cores of the ARMv7 processor, and close to 20 minutes on a single core (without ccache). The test has been done using ArchLinux ARM with a micro SD card of class 10 (said to be able to manage 50MB/s in write, 60MB/s in read). In comparison, the buildfarm machine hamster, which is a Rasp PI 1, takes close to 2 hours to do the same operation, using the same OS and a similar class 10 card (30MB/s but the I/O is not the bottleneck).