Is there an easy way to debug and find out why a query runs slow?
Thanks to mrdenny! Your commment pointed me in the right direction!

I've pasted the query into SQL Management Studio

Then used the option Display Estimated Execution Plan

This gave me an output that an index in one of the tables belonging to the view needed to be recreated, so I deleted it and create it from scratch using the suggested output.

CREATE NONCLUSTERED INDEX [&lt;Name of Missing Index, sysname,&gt;]
ON [dbo].[MemberOrders] ([Client_Id],[Project_Id],[Person_Id])


Now the query takes only 2 seconds and not the 1:30min that it used to take! 

Thanks a lot guys