After Effects normally taxes your Mac's resources heavily, especially when you don't have a lot of memory. Using renice on the processes related to After Effects can greatly improve the performance of other applications you may be running.
By using renice on the processes related to After Effects and some others related to the system your system will be more responsive. Just launch this script after launching After Effects and render a few frames (RAM preview, render). The script will renice the priority of the After Effects processes, and some others that are related.
And then you may run iTunes, Dropbox, and other programs without problems while working with After Effects.
tell application "Finder" to set theApps to name of every process
activate
set newPriority to "15"
set theApp to "After Effects"
set thePID to do shell script "ps -axww | /usr/bin/grep '[/]" & theApp & "'| awk '{print $1}' | head -1"
Renice(newPriority, thePID, theApp)
set newPriority to "15"
set theApp to "CS5.5ServiceManager"
set thePID to do shell script "ps -axww | /usr/bin/grep '[/]" & theApp & "'| awk '{print $1}' | head -1"
Renice(newPriority, thePID, theApp)
set newPriority to "15"
set theApp to "Adobe QT32 Server"
set thePID to do shell script "ps -axww | /usr/bin/grep '[/]" & theApp & "'| awk '{print $1}' | head -1"
Renice(newPriority, thePID, theApp)
set newPriority to "15"
set theApp to "iTunes"
set thePID to do shell script "ps -axww | /usr/bin/grep '[/]" & theApp & "'| awk '{print $1}' | head -1"
Renice(newPriority, thePID, theApp)
set newPriority to "15"
set theApp to "Dropbox"
set thePID to do shell script "ps -axww | /usr/bin/grep '[/]" & theApp & "'| awk '{print $1}' | head -1"
Renice(newPriority, thePID, theApp)
set newPriority to "15"
set theApp to "aeselflink"
set thePID to do shell script "ps -axww | /usr/bin/grep '[/]" & theApp & "'| awk '{print $1}' | head -1"
Renice(newPriority, thePID, theApp)
set newPriority to "15"
set theApp to "kernek_task"
set thePID to do shell script "ps -axww | /usr/bin/grep '[/]" & theApp & "'| awk '{print $1}' | head -1"
Renice(newPriority, thePID, theApp)
on Renice(newPriority, thePID, theApp)
try
set theConfirmation to (do shell script "renice " & newPriority & " -p " & thePID)
on error
set theConfirmation to (do shell script "renice " & newPriority & " -p " & thePID with administrator privileges)
end try
end Renice
Mac OS X Hints
http://hints.macworld.com/article.php?story=20111206043349948