SQL Server 2012 CPU usage spikes to 100%
What is the exact type of the user_id column?

It is very likely not a VARCHAR column therefore the rules of data type precedence dictate that the comparison must be done by casting the user_id to the type of the '10' literal, thus negating the possibility of an index seek and forcing a full scan. This is a very common mistake.

I recommend reading How Data Access Code Affects Database Performance (do a mental translation for Java - Hibernate) and How to analyse SQL Server performance.