Until this beta (haven't had a chance to try b10 yet), parts downloaded in a "sane" fashion. if you had a main server and a fill server(s), it would queue up X parts (X being threads for main, say 8 for me) and download pieces in order.
The behavior I see in b9 is that it seems to queue up a huge number of parts 30-40-50? (perhaps a sum of all the backup server threads?), and it download the parts from them in a haphazard order (i.e. file 3 has parts going before file 2 is anywhere near done). i.e. of course it tries to download the first part from a file first, but then file 2 or 3 might finish that race first then it gets entirely queued up, so even once file 1's part 1 has finished, it doesn't start downloading for a bit.
guessing this has to do with the new priority system? see some things in b10 log that might indicate it's been seen/fixed, but figured no harm in asking.