Running two jobs at once is slow
 

I have a 20 core machine (20 real cores, 40 with hyperthreading). If I run a particular job with 10 cores it takes about an hour. If I open a second terminal window and run the same job with 10 cores, each job takes about 2 hours. It seems to me like each job should take about an hour, since I've got 10 real cores for each job. Any idea what is going on?