I think AppleScript requires a different approach.
set folderAlias to choose folder
set elapsedTime to (current date)
tell application "System Events"
set folderList to (every file of folderAlias) # whose visible is true)
set elapsedTime to ((current date) - elapsedTime)
set itemCount to (count items of folderList)
"items = " & itemCount & return & "time = " & elapsedTime & " sec"
display dialog the result buttons {"OK"} default button 1
end tell
513 items, 0 secs
and with "disk items"
606 items, 0 secs
So the distinction is really in the clause "whose visible is true"
For the Finder without that clause I get similar results:
items = 511, time = 0
(10.6.3, MacBook 2.2 GHz)
For the current example, I would use
which gives a time of 1 or 0 seconds on a couple of hundreds and 6 secs on 8500 files, where I tested
if myDate > cutoffTime
with cutoffTime = ((current date) + (1 * days))
Since "entire contents" goes into subfolders, I think this is still acceptable.