Hi there.
In the last months on my site (Idolium), I observed that in the logs there was a lot of "access denied" items, all made in a few seconds from the same IP. The "user field" was full of strange names, that do not correspond to any registered user. Like this (from today):
user 22/04/2013 - 19:01 Login attempt failed for larickjgd. Anonymous
user 22/04/2013 - 18:42 Login attempt failed for NiniBronsorge. Anonymous
user 22/04/2013 - 16:26 Login attempt failed for Weipscreese. Anonymous
user 22/04/2013 - 11:58 Login attempt failed for crynctymn. Anonymous
user 22/04/2013 - 06:48 Login attempt failed for hememaillaHew. Anonymous
user 21/04/2013 - 20:47 Login attempt failed for hojjanama. Anonymous
user 21/04/2013 - 15:19 Login attempt failed for hojjanama. Anonymous
Obviously, all this is an spam try. But, if your server is not powerful, or your site is a bit slow, any request to your site makes it slower yet. And, of course, your log is full of garbage.
How can I avoid this. Well, my site has Mollom installed for registering a user, so for bots is hard to register as a user. And usually this bots use strange names to avoid duplicate a usual username, so, when mi site emails me with a new registered user, I can suspend the account if I suspect from the strange name.