hmmm what version are you running ? running latest here on galaxy s4 and have no noticeable lag when launching an app or going back using home button.Actually running really smooth here.Have you tried uninstalling and reinstalling,maybe it is rom related and not themer related.
Remember we are still in beta and there are still some problems,I have had many problems here with previous releases but still found it more adaptable thab go in my opinion.
The Lord of the Blings
One theme to rule them,one theme to find them
One theme to bring them all,and in the darkness bind them.
Over one hundred themes added to themer app