there is limit on the length of the edit string (I set it to 1.5M it is enough to contain maybe 40000-50000 group names) it is more than feasible to get full results in reasonable time.
the xpat command is not implemented effectively by servers, it is issed on per newsgroup basis, so the less newsgroups you look in the faster it will be (even for one newsgroup it may be quite slow, but also for empty groups where for sure there are no results - replying to any command by a server is accompanied by a delay).
you can sort newsgroups by the number of articles and extract relevant newsgroups (through edit menu->message filter) first to reduce the number of newsgroups to look in.
also check
http://alt.binaries.nl you can look there and then to launch xpat for a specific newsgroup.