Hi!
If you deselect custom fields, then the search speed is normal? (3-5 seconds)
What is the size of your databse? (I mean number of posts, products etc..)
In case of bigger database (more than 10-20000 records) the search speed highly depends on the server speed and the configuration. There are a few things you can try first.
- Try to disable the relevance http://i.imgur.com/3CvQi6B.png
- Turn off the fulltext search: http://i.imgur.com/1BcUDJ5.png
- Lower the count of the results below 10: http://i.imgur.com/3sv1LLH.png
- If the content and the excerpt does not contain text, then disable the search in them: http://i.imgur.com/e9PdMRV.png