worth mentioning as well, back in the day there would often be bitcoin mines setup with rigged power out in the expanse of china.. with the WAN link being an old 2g mobile phone.
the max send rate on 2g being around about 0.05-0.1megabit/sec (so 5-10kilobytes/sec).

with even a half megabyte block taking up to 1.5 minutes to get out onto the network, you can see why blocks with only the coinbase txn were mined.

james