did you ever try another dashboard and transferring files? ... i recommend UnleashX in terms of pure speed as from my own personal experience UnleashX maxes out my router @ 10MB/s ish ... avalaunch is noticeably slower for me than UnleashX is (although if you use QUIX your best bet is to stick with avalaunch as QUIX seems to work best with it, especially if your making a ISO file from the xbox to the pc) .... XBMC is nice but it's ftp is slower in general due to it using more CPU cycles so that right there automatically guarentees it to be slower than dashboards like UnleashX etc.
just run unleashx as a application and see what happends
... if it still acts up try another ftp client like FileZilla (which is free) ... me personally i like FlashFXP the best when it comes to ftp related stuff.