Hi,
I am not sure I replicate anything like that, it does run smooth on my tests.
Also, make sure you test on the device, Maybe it's not smooth in editor but totlaly fine when built.
Indeed, iTween is an very old library, you should consider something more appropriate, like doTween or even the new version of iTween ( by the same author, pending playmaker support, so useable if you don't know how to script).
If you have a test scene where it doesn't run smooth, send it over.
Bye,
Jean