Quoted from the wiki of BitComet:
To get the best speeds, BitComet needs an open listening port, just as most other P2P applications do.
An yellow light means BitComet successfully requested the probe, but did not detect it at your listen port. This indicates that the port is blocked by one or more firewalls. Your client can therefore reach other clients but other clients can not initiate contact with your client.
If your listen port is blocked you can still transfer files with BitComet, but your transfer speed probably won't be very high.
No listening Mode: will greatly slow down your upload/download rate. Not recommended. (Exceptions are those who are situated behind routers or firewalls which they cannot access and configure. For them, this setting will provide the means to still be able to download even though at much lower speeds).
E.g. I can't not configure the outermost firewall, so the listening port used by a P2P client is blocked, thus I can only be a client not a host. But how would it greatly slow down the download speed? I can still connect to all the hosts as a client.
Is it merely because I would lose those client peer connections if I can be a host?