SQL Server slows down on one expensive query I have a Sql Server 2012 with lot of CPU and memory.
When I run a query with a bad plan the server slow down and all other query, even of other databases, become very slow.

When I run the query:
- the CPU remains under 14% - so is not a CPU problem
- there are not grant memory pending - so is not a workspace memory
- non problems on disk I/O - so is not a disk I/O problem
- PLE is 8000 - so is not a buffer pool problem

I notice that the query use a lot a workspace memory (probably reaching the 20% limit) and free memory goes near to 0.

I'm stuck.

Could so one help me?
When I run a query with a bad plan the server slow down and all other query,


That says they're contending for the same resource, and 


  non problems on disk I/O


doesn't say much about why the disk wouldn't be the resource they're contending for.  

Logically we know that a query with a "bad plan" is going to read a lot of data from the disk, perhaps scanning one or more tables, because the definition of a "good plan" is one that uses indexes and minimizes I/O.  So your first suspicion should be that query is bandwidth-limited, and your second guess should be that the I/O cache is being flushed by the bad query.  

If you check your I/O queues in SQL Server when the bad query is running, I expect you'll see the other queries have non-zero queue lengths.