How to increase the speed of SQL Views queries?
Take the query from the Views admin and from the mysql command line, or phpmyadmin add the EXPLAIN keyword in front of it. Assure it's using indexes on query criteria and that the query is optimized.

If it's not a fully optimized query then simply add an index to your SQL schema as needed until it's as optimized as it can be.

You can either add the index directly to tables through phpmyadmin, or create a module to index the necessary db fields (and then drop indexes if the module is disabled).

Drupal ships with a db that is not fully optimizted for any query you could generate with views. Module creators make general assumptions in the table they make to provide primary keys for example -- but they don't index every column of their schemas as taht would be wasteful. How you use Fields on your site is different from how I would use fields on my sites ... etc.

1 module that may help you index your Drupal fields (after evaluating the EXPLAIN output is) Field Index:


  Field modules don't always add indexes for its columns (value column
  of text field for example), this leads to poor performance in views
  when we would like sort or filter views using the unindexed columns.
  We can check the query executed by views and add custom indexes
  manually, but it's not easy to deploy.
  
  With field_index module, we can create custom indexes for fields. The
  indexes are exportable.