ok, wasn't aware of what this "App Nap" you referred to was. Googled it and disabled that.
Yes, that fixed the issue of the App pausing when is minimized in the toolbar when running.
However, it is still VERY slow sending mail under Mavericks even when the active app on the desktop.
It is taking at least 3 times as long to send my list as before Mavericks upgrade. This is running on a recent iMac. The verification for me that it is something with Mavericks is that when running on a 4 your old Macbook running Mountain Lion, not Mavericks, it runs quite fast as usual (using my send grid smtp service)