We are having an issue with SmartCrawl. It is causing the publishing of posts to stall. When SmartCrawl is active, publishing a post results in WP returning the Connection Lost error. Generally, when SmartCrawl is active, it takes WP about 2 minutes or so to publish a post
I've been working on a staging site to test it through and the site has been scaled down to only a default theme and SmartCrawl, with AutoLinking turned off. The only complicating factor remaining on this site currently is the large database. It has 20,000 standard posts and 8,000 additional posts of a custom post type (which not being used since the plugin creating them is deactivated as part of this testing.)
My suspicion is that SmartCrawl doesn't play well with large databases, which could be considered a major problem for an SEO plugin.
The site is located here: http://staging.elastic-horse.flywheelsites.com/
It is a staging site and it is HTML auth blocked. Login with user: flywheel and pass: current-lift
Support access is activated in WPMU DEV Dashboard. Feel free to login and test as you need too to confirm my findings, but I suspect that large databases might be a weakness for this plugin.