Hi,
Thank you for your kind words!
Well, the plugin does not have an upper cap of what it could handle, at this point this might be related to server performance. Indexed queries should still be perfectly fine, your SQL server should be able to handle them.
First I would strongly suggest trying the tips from the fine tuning documentation, especially chapters 1-2.
Aside from that, in your case I have a few more suggestions, which may have actually a great impact on the overall performance:
– If you are using any search filters, I suggest turning them off if possible. Not only visually, actually removing any filters: https://i.imgur.com/PlrkxKq.png
Filters may require table joins or subqueries, which may slow down the query, or SQL may use a non indexed query, which actually can take some time to execute. This can have a really big impact on server usage an performance.
– Using only a primary logic, and not a secondary. In your case, probably the “AND with exact word matches” is a good option, due to the great amount of data to choose from: https://i.imgur.com/ajm9bEL.png
– Running an index table defragmentation will also help: https://i.imgur.com/6VABYng.png
If everything is set up similarly, I think you should expect a decent response time, somewhere around or below 3 seconds if the server is not too busy. But of course this greatly depends on the number of plugins, their code quality – a great disatvantage in worpress is, that in each ajax request the core, all the plugins, themes and everything needs to load up, which add to the response time – usually a lot, sometimes even seconds while everything loads up.