Good news. I finished a possible release candidate for version 1.4, I'm attaching it to this reply for testing. I have fully optimized the database queries based on the table indexes and best practices from the MySQL manual. From this point on I'm almost sure the performance only depends on the configuration and the server speed.
After installing, please change the Maximum Keywords count option to 4 on the Relevance Options -> Keywords options panel: https://i.imgur.com/YDEJ0tb.png
I have found that 4 is more than enough and optimal for the performance.
Let me know how this one works!
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