Actionscript in the right hands can perform very well. A big part of the problem lies with the barrier to entry (low) and the features available to developers and designers via its API and tool-set.
Things like bitmap manipulation and real-time filters (drop-shadow, glow etc), they are really easy to implement and designers love the effects that can be achieved and tend to design with them in mind (can't blame them really), but without optimization care and skill it will eat a CPU alive.
These effects can also be implemented without a single line of code, a lot of the time it is not even the developers at fault.
I worked agency level for a number of years before I lost the will to live and gave it up, I seen this kind of thing daily and the culture of getting it out there didn't help either, unless there was a really switched on PM or lead developer in charge who understood this optimization time was generally second class.
EDIT: it did not help that OSX was a second class citizen and Flash took a 15% hit in Firefox (windows) either.
Things like bitmap manipulation and real-time filters (drop-shadow, glow etc), they are really easy to implement and designers love the effects that can be achieved and tend to design with them in mind (can't blame them really), but without optimization care and skill it will eat a CPU alive.
These effects can also be implemented without a single line of code, a lot of the time it is not even the developers at fault.
I worked agency level for a number of years before I lost the will to live and gave it up, I seen this kind of thing daily and the culture of getting it out there didn't help either, unless there was a really switched on PM or lead developer in charge who understood this optimization time was generally second class.
EDIT: it did not help that OSX was a second class citizen and Flash took a 15% hit in Firefox (windows) either.