True! If you had as an example an entire house code used for lights and wanted to turn all of them on at the same time, there are 3 approaches that yield wildly different implementation times. This assumes a script taking no time, and perfect reception on your PLC without interference.
Manual: (hail module, turn it on, move to next)
command cycles time cumulative time
E1 25 0.417 0.417
E1 ON 25 0.417 0.833
E2 25 0.417 1.250
E2 ON 25 0.417 1.667
E3 25 0.417 2.083
E3 ON 25 0.417 2.500
E4 25 0.417 2.917
E4 ON 25 0.417 3.333
E5 25 0.417 3.750
E5 ON 25 0.417 4.167
E6 25 0.417 4.583
E6 ON 25 0.417 5.000
E7 25 0.417 5.417
E7 ON 25 0.417 5.833
E8 25 0.417 6.250
E8 ON 25 0.417 6.667
E9 25 0.417 7.083
E9 ON 25 0.417 7.500
E10 25 0.417 7.917
E10 ON 25 0.417 8.333
E11 25 0.417 8.750
E11 ON 25 0.417 9.167
E12 25 0.417 9.583
E12 ON 25 0.417 10.000
E13 25 0.417 10.417
E13 ON 25 0.417 10.833
E14 25 0.417 11.250
E14 ON 25 0.417 11.667
E15 25 0.417 12.083
E15 ON 25 0.417 12.500
E16 25 0.417 12.917
E16 ON 25 0.417 13.333
collective: (hail all modules, turn all on with single ON command)
command cycles time cumulative time
E1 25 0.417 0.417
E2 25 0.417 0.833
E3 25 0.417 1.250
E4 25 0.417 1.667
E5 25 0.417 2.083
E6 25 0.417 2.500
E7 25 0.417 2.917
E8 25 0.417 3.333
E9 25 0.417 3.750
E10 25 0.417 4.167
E11 25 0.417 4.583
E12 25 0.417 5.000
E13 25 0.417 5.417
E14 25 0.417 5.833
E15 25 0.417 6.250
E16 25 0.417 6.667
E ON 25 0.417 7.083
native: (utilizing the all on command)
command cycles time cumulative time
E All ON 25 0.417 0.417
So, with the worst case implementation it could take over 13 seconds to turn all lights on. HG uses the second approach and it could take 7 seconds. If the native X10 command were implemented, it would take ~0.4 seconds to turn on the lights for a house code.
EDIT: I asked a similar question at the X10 forums 2 years ago and was given a very detailed response.
http://forums.x10.com/index.php?topic=28743.msg160924#msg160924