In short: It is faster to simply select taxonomy terms and process them within PHP to find possible similar keyword matches – because they are more likely to be very short words. It works nicely even for cases with over 50 000 terms. In case of titles the table is much bigger, and most titles are multiple words long, which exponentially increases the comparision process. Not to mention the possible filter criteria, exceptions during the initial select query etc.. It would not be an issue whatsoever if the processing power and memory on regular hosting servers was much greater, but we can not risk that unfortunately. We try to limit the plugin to run on the customers local server instead of charging monthly fees for external processing services.
This feature will be improved for sure in the future as we are adding improvements and new features. Do not worry, the solution for the terms was added not too long ago, and it seem to work really great in comparision how it was before. I’m sure we can find a way sooner or later to somehow use that on titles without the performance issues.