Thanks Nigel for the revised testing method.
Just for the sake of completeness, I put your script in post 2 to the test. I first ran your initial script in post 20 to set a baseline and the result was 0.030 seconds. I then tested your script from post 2 (modified to use reference-to operators) and the result was a very respectable 0.055 seconds.
use AppleScript version "2.4" -- OS X 10.10 (Yosemite) or later
use framework "Foundation"
use scripting additions
set Test_List to {"217.0 241.0 217224033 37.404", "225.0 241.0 057181074 25.407", "249.0 241.0 193039045 11.751", "241.0 241.0 247147030 35.026"}
set sample to Test_List
repeat 999 times
set Test_List to Test_List & sample
end repeat
set sample to missing value
count Test_List
say result
set start to current application's class "NSDate"'s new() -- Start timing here.
set Result_List to {}
set Result_List_Ref to a reference to Result_List
set astid to AppleScript's text item delimiters
set AppleScript's text item delimiters to space
repeat with this_Item in (a reference to Test_List)
set end of Result_List_Ref to text item 3 of this_Item
end repeat
set AppleScript's text item delimiters to astid
return "That took " & -(start's timeIntervalSinceNow()) & " seconds."
--> "That took 0.055496931076 seconds."