Performance issues

[xoolive]

According to this graph, the decoding time if an order of magnitude faster than the throughput time.

Yet there seems to be some drift in the reception of messages (i.e. system_timestamp - gnss_timestamp) when we decode many receivers: here only one receiver is drifting, but in daytime it may be all of them.

It is a bit hard to investigate right now (maybe to be transformed into an issue)

DF20 and DF21 (about 25% of the received messages) are slower to decode because their payload is of an unknown type.

Originally posted by @xoolive in #126 (comment)