The client and the DCC protocol are not made for that kind of heavy usage.
That's a load of bs.
That's not a load of bs, clearly you know nothing about the DCC protocol, the redundant ACKs it sends, its non-existent support for large packet sizes which increase throughput on dedicated file sharing/transfer protocols, or the primitive nature of the DCC resume mechanism within mIRC. Not to mention the absence of any integrity check (beyond TCP's own, which is weak and is not sufficient to guarantee integrity of large files).
mIRC and DCC may or may not have any specific effect that another file transfer protocol wouldn't, there's no way for
anyone here to know that with the information we've got, but if you think DCC is anything resembling a decent dedicated file transfer protocol you are very much mistaken.
And it's entirely possible as Mentality said that the ISP are identifying the use of DCC and capping it, as many ISPs do for file transfers. Again, there are many
dedicated file transfer protocols which support protocol obfuscation which prevents the easy identification of a protocol and prevents it from being capped by an ISP.
