I have been a happy user of Ajax Search Pro for about a year. Sadly, I had to uninstall it recently for two reasons (which I hope are addressed in future updates).
The first is that the plugin was seriously slowing down my site load times. Looking at my newrelic reports, 40% of my load time was due to the php that is loaded by the plugin, which amounted to 2 seconds in total. This was strange, as I’m running an M3.2Xlarge instance on Amazon, so it should cope just fine with the requirement. So, I don’t know what’s going on, but code optimisation of some kind could be required.
The second issue was that my log was getting flooded with errors generated by Style8.css and Style9.css. I only had one of those in use, so I’m not sure why the server was forced to load both. In any case, they were generating tons of errors and I couldn’t catch other errors that might be more important to me.
At the end of the day, you really have the only comprehensive plugin out there for site wide search that includes buddypress and bbpress, so I hope to use the plugin again some day. Good luck!