Also (copy pasting from another post of me):

SC supports gzip HTTP transfer compression. If you tell your fastdl webserver to compress the mime types "text/* audio/* image/* application/octet-stream" this should compress all custom content (application/octet-stream is used for files with unkown mime types like wad/bsp/mdl assuming you have not added any custom filetype/mime associations). Also use a reasonable minimum filesize (like only compress files >4KiB). mod_deflate for lighttpd or apache can do this (on the fly compression). But note that compression uses CPU power, especially when the webserver is running on the same host as the gameserver (think 20 players on mapchange to a new map with loads of content). Compression level also matters. Traffic-wise with such a configuration it is about the same as fastdl on Source games, just that gzip instead of bzip2 is used.