I've been working my way through some of the CMU free .bvh's and had similar delays importing. Anything over about 2mb is slow to load, get to 3mb and Poser appears to go into hybernation. Anything bigger and I swear she's waiting for a kiss from a prince with a decent set of hedge trimmers.
I opened these bvh in Wordpad to amend the hierarchy to suit the figures I intended to use and noticed that the documents ran into hundreds of pages, so I can understand how all that info may take time to load. 
My point though, is that it may be better to break your animation down into segments, then export  more user friendly chunks. (Save it as a sequence of animated poses, then export those as bvh)
It would make life easier too when it comes to rendering, as has already been mentioned Poser seems to have its own "soft" limits. I find that after rendering about 2000 frames non-stop it gets very slow and eventually locks up so I tend to work in smaller chunks and restart the programme between segments to clear buffers or whatever is clogging up the system.