I think there is a bug with HTTP download that sometime fails and fall back to slow download mode.
I think It's on the client side because the transfer initially starts well but at some point it fails.
Sometimes its working, sometime it's not (same map).
I also had this bug in genuine COD5... so may be not plutonium related, but could be fixed anyway ? Or better explained, maybe its an incompatibility with apache2 ?
Tonight I will try to wireshark it..
Note: I remember this bug was also in COD4(x) some time ago but today no more problem.
Note2: you could try connecting to my server (CHACHA DM) which is full of custom maps with fast DL.
Please do provide the parts of your server config that are related to fastdl.
set sv_allowdownload "1"
set sv_wwwDownload "1"
set sv_wwwBaseURL "http://newchacha.ddns.net/games/cod5/"
set sv_wwwDlDisconnected "0"