performance difference between exec of SP and running the statements

I am experiencing a huge performance difference when calling a stored procedure with EXEC (with parameters) or running the exact same statements inside the stored procedure separately in Mana开发者_开发知识库gement Studio. The EXEC is 3-4 times slower. What causes this?

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See this answer.

It's a phenomenon called "parameter sniffing" - basically SQL tries to create the optimal execution plan based on what it expects your parameters to be, which is often not accurate at all.