Views Long query execution time serious problem
There is no one line answer for you question as there are multiple things that can affect a query running slowly.

What you should do is enable slow query log in MySQL and try to debug from that point why your query is taking so long to execute. Things you can do are,


Tune your MYSQL.
Turn on slow query log.
Use the Explain command to find more info on how your query is executing, http://dev.mysql.com/doc/refman/5.7/en/using-explain.html.
Watch out if the tables are correctly indexed.
Last but not least, you can use hook_views_query_alter if you feel views is not creating the query optimally.