Spam BLIP stops comment spam before it is posted, using DNS blacklists, existing comments marked as spam, and user defined lists.
Spam BLIP stops comment and ping spam from being posted, primarily by checking the IP address attempting to post a comment in one or more of the public DNS blacklists. A number of options are available to refine the check, and with the option defaults, a DNS lookup is only performed the first time an address attempts to post a comment; thereafter, the address might quickly 'pass' because it was not listed, or quickly be rejected because it was listed. Spam BLIP creates, and maintains, a database table for this purpose, and database lookups are quite fast. Therefore, concerns about DNS lookup time can be limited to an initial comment attempt.
Here are some features of Spam BLIP to consider if you are not yet falling over yourself to get it installed:
When WordPress is producing a page for a visitor, it checks whether comments are open for each post, and it allows plugins to "filter" the check. Spam BLIP uses that filter, but does not do DNS lookups at this stage, because DNS lookups can take perceptible time. Spam BLIP does check optional user-set black and white lists, and optionally existing comments that are marked as spam, and of course Spam BLIP's own database records. Those checks are fast, so they should not have a perceptible effect on page loading. Furthermore, on pages with multiple posts, WordPress runs the filter for each, but Spam BLIP stores the first result, so even the fast checks are not repeated.
When a comment is actually submitted, Spam BLIP does the above checks, then the DNS lookup only if necessary. At this stage, if the DNS lookup causes a perceptible delay, a real human (or very clever pet) making the comment should perceive it as mere server-side processing. As for spammer robots . . . let them wait.
Spam BLIP comes configured with blacklist domains that have worked well during development, so a user should not need to be concerned with the blacklists, but there is an advanced option to add or delete, activate or disable (yet save) list domains, and configure the interpretation of a return from a successful lookup.
Spam BLIP provides user-set whitelist and blacklist options.
Spam BLIP provides options to check for pings/trackbacks, and for user registrations. (The option to blacklist-check user registration is off by default. See "Tips" under the help tab on the Spam BLIP settings page.)
Spam BLIP provides options to configure a 'Time To Live' (TTL) for its database records, and a maximum number of records. The TTL is important because, generally, an IP address should not be marked permanently. Consider an ISP that quickly disables any account that is found to be spamming. An honest ISP is also a victim of spammer abuse, and will need to reuse addresses. DNS blacklist operators provide means for IP address owners to get records removed -- Spam BLIP provides a configurable TTL for its records. (Database table maintenance is triggered approximately hourly by a WordPress cron event.)
Spam BLIP will optionally check if a commenter address is a TOR exit node. TOR (The Onion Router) is an important protection for people who need or wish for anonymity. You may want to accept comments from TOR users (you should), but unfortunately spammers have exploited and abused TOR, which has led some DNS blacklist operators to include TOR exit node addresses whether or not it is known that the address is spamming. If you enable this option (you should), it might let some spam get through. In this case, mark the comment as spam, and use the Spam BLIP option to check existing comments marked as spam; or use Spam BLIP in concert with another sort of spam filter, such as one that analyzes comment content. (Please report any conflict with other, non-DNS blacklist type spam plugins. Note that Spam BLIP is not expected to work in concert with other DNS-type anti-spam plugins.)
Spam BLIP includes a widget that will show options and records information. The widget might or might not be an enhancement to your page, but in any case it should provide feedback while you evaluate Spam BLIP, so it might be used temporarily.