the amount of members has no effect really on the speed of the site. its just the amount of visitors on the site ata time, and how ofter there refreshing a page. The server and database can't hand't that many requests, so they get put in a que. Thtats why when you click a link someimtes it just sits there for a long time. But when the site finaly starts loading, it loads really fast.
A lot of the members signups are from people downloading stuff, you need an account to download. This is to stop total rapage of bandwidth from the site, and with the hope they might come and post. I know from stats that thre a lot of ppl who read the forums that don't post.