Hi,
Okay, so you have over 50 000 posts there, and the index table was 5 million records long, that is huge. The server was simply not able to process the query on such a huge amount of data.
I have found a possible configuration that will likely work the best for you:
– First, I have cleared the index table, so that no unneccessary records are being stored.
– Then I changed back to the regular engine, and only enabled the search in titles: https://i.imgur.com/elE1P5t.png
The post title fields in WordPress are indexed already, that is why I decided to switch from the index table. And since you have over 50 000 posts, searching the contents is not neccessary, as the titles will almost surely yield enough results.
– I also disabled the secondary logic, to gain even more performance boost: https://i.imgur.com/5p2a2x9.png
Now it yields almost immediate results.
This is a really solid configuration in my opinion. If you want to switch back to the index table engine, you can – just make sure to re-create the index, and only enable to index the post titles – as the database will not be able to process the query for 5 million records in the content fields, it is too much for the server to handle.
I have also temporary disabled the log and query log plugins you had. I have re-enabled them now. I strongly recommend turning those off once you are done with debugging, they can impact the server performance negatively.