I have possibly found the most optimal configuration based on your description. I think this will be the fastest:
– I have re-created the index table
– Then I enabled the index table for the 'Main search'
– Then I disabled the product and post categories from results: https://i.imgur.com/AylqNPZ.png
– I have also disabled the keyword suggestions: https://i.imgur.com/HxWHs5t.png
There is one other thing I have noticed. You have a lot of product categories excluded from the search, that might affect the perfromance a bit, but that should be fine.
Currently I am seeing a much better performance: https://youtu.be/cj0adl_kUEA
The tests are around 1.5-2.5 seconds, which should be much better now.
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