A lot depends on the amount of 'work' that the swf is doing. The more that's going on, the harder the job is, and the slower it will run on less capable machines. Some of the things that are particularly 'stressing' on a system are.... (in no special order)
* use of transparency (alpha) particularly when there are multiple semi-transparent objects overlapping one another
* scaling items (again, multiple items scaled simultaneously = harder work)
* 'particle' systems (multiple - perhaps hundreds or thousands - of small shapes being moved using actionscript) e.g. explosions, snow, rain, bubbles etc
* complex or lengthy mathematical computations (eg trigonometry or other 'floating point' math).
The best way to 'tune' your swf is to create a 'speedometer' that tells you the number of frames per second that the Flash Player is achieving. Then remove one part of the swf, and re-run it to see how the removal of that effect / movieclip affects the speed. See advice on how to build that speedometer at
http://www.moock.org/webdesign/flash/actionscript/fps-speedometer/
