In that picture I cannot see the process taking up the total of the first CPU Core - if you look, Processor 0 is up rather high, which could cause a process to slow down.
Also have a look if your RAM or Disk usage is maxing out somewhere.
But, it could also be a different issue and it just seems that your pc is slowing down - input lag or what ever - might be that just the GPU is being stressed for some reason, or even nothing related to all that.
So basically, you need to look through all that, see if you find anything that is taking up the resources and then try to figure out why (which is quite a job to do).
You could also try the event viewer in windows (the name is language specific its the eventvwr.exe in
windows/system32/ and look if you see any Events that pin point the issue.
Also you might try dxdiag - just for the sake of it and see if it pin points anything.