SQL Server slows down on one expensive query
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.