Hey Alex and Paul,
thank you @actech for pointing out the relation to WIFI. I can confirm that. That lead me to further testing with the result, that the compiling time is equivalent to the WIFI upload time
@paulmw: This might be interesting for you (if you did not know yet 
)
I checked two projects with different number of blocks and made 3 times a little dragging on a WHEN-block in the block-section:
my large project with 700 blocks: obeyed compilation time about 13 seconds
small project with 20 blocks: nearly no compilation time
You cannot use the total received and total sent values, because I listened to spotify while thunking. But if you look at the sent curve it says for the large project a total of ~2 MB (area under the curve: 200 kB/s * 10 s) is sent via WIFI on each slight change!
My calculations are: if my WIFI / DSL connection is limited to 2 Mbits/s, it takes 10 s for 700 blocks.
→ Each Mbit/s speed of your DSL connection lets you compile 35 blocks in one second.