By large I mean 800KB or so and we're looking at processing times upwards of 45-60 minutes. Inside each CSV is 400 posts, it uploads it nice and quick and then the server just gets slammed.
I running the blog on a quad-core xeon 2.8ghz machine with 2GB of ram and my load averages are around 3.0 2.0 2.0 with mysql sucking up around 80% of the cpu power the entire time the import is going on.
However if I take a smaller file, around 200kb with 100 posts it'll get finished in 20-30 seconds no problems. Even if I import the smaller file while the larger one is going on it still completes in around 30 seconds.
So it seems like there is a critical mass for the CSV that when reach melts the plugin into a pile of goo, slowing the importing process to a crawl. I know it's not the number of posts because i've taken a CSV file with 1400 posts that was only 200kb and it finished in 30 seconds or so which leads me to believe it's the size of the file that is causing this mayhem