performance a little sluggish/jerky on older phones?

Games
#1

I have had some people testing my game and whilst generally fine one comment is on the older phones (e.g. iPhone 4) it is a little sluggish & sometimes physics objects go into the wall a bit rather than bouncing off.   I was interested in general feedback from anyone creating physics based games and performance on “older” phones.

Q1 - Is it normal to see some sluggish behavior on slight older phones for physics based games: e.g. IPhone4, or the Samsung GT-I9000 2.2 I have?    

Q2 - Here are the stats I have from the simulator if this helps - any comments.  Note the different levels would trigger differ levels of resolution usage for images/sprites.  From my readings these should be ok.
 

 - Model  / fps / memory / texture_memory

 - iPhone: 30, 1811kB, 9.6MB

 - iPhone5: 30, 1874kB, 34MB

 - iPad Retina, 30, 1800kb, 119MB

Q3 - Just noticed I am defaulting to 30 fps, should I increase this to 60 fps to assist?  Or would this make things worse on older phones.  

Q4 - Note that I don’t use the physics engine, but my own so to speak.  To cut a long story short I time the period in between cycles (enter to gameLoop) myself and then use this time in working out new positions (e.g. new position = current position + speed * time etc).   So if the phone slows it doesn’t really slow the game.  Not sure if this is good or bad overall, but just how I did it.  Wondering if using Corona “time between” cycles I’d get the same result anyway and this wouldn’t help.

Just any general comments that help me determine whether I need to do something, or just accept running a Corona physics based game will need the later phones available to be snappy?

#2

PS
Q5 - how do I measure how well I am doing re performance optimization? Would dropping FPS be an indication of this? Or do you need to measure CPU somehow?

Re optimizations I mean like: http://www.coronalabs.com/blog/2013/03/12/performance-optimizations/

PSS (Edit):  Did run the debug version of the app on the Samsung GT-I9000 & noted the FPS did not drop below 30 (i.e. it stayed on 30).  Yet the gameplay does seem just a touch sluggish, so I’m not sure how you would measure/quantify this in a test however then???      I did have a look on the PassMark benchmark here however and note this model has a benchmark score of 995, whereas the latest Android devices seem to be 3000 - 5000 range.  

#3

PS
Q5 - how do I measure how well I am doing re performance optimization? Would dropping FPS be an indication of this? Or do you need to measure CPU somehow?

Re optimizations I mean like: http://www.coronalabs.com/blog/2013/03/12/performance-optimizations/

PSS (Edit):  Did run the debug version of the app on the Samsung GT-I9000 & noted the FPS did not drop below 30 (i.e. it stayed on 30).  Yet the gameplay does seem just a touch sluggish, so I’m not sure how you would measure/quantify this in a test however then???      I did have a look on the PassMark benchmark here however and note this model has a benchmark score of 995, whereas the latest Android devices seem to be 3000 - 5000 range.