Is the WOL packet supposed to contain a string of hex values of 0xFF or is it supposed to contain text "FF FF FF FF FF FF"? If the later is the case, then the hex byte content of the string you suggest is:
0x46 0x46 0x20 0x46 0x46 0x20 0x46 0x46 0x20 0x46 0x46 0x20 0x46 0x46 0x20 0x46 0x46 0x20

whereas I would expect that the packet likely needs to be 6 byte chunks as:

0xFF 0xFF 0xFF 0xFF 0xFF 0xFF.

Which in Javascript string representation is:

'\xFF\xFF\xFF\xFF\xFF\xFF'.

Maybe this is causing the delay? Maybe it is just dumb luck that this works and the PS3 is simply responding to something instead of wanting an exact WOL packet.

I also did a bit of searching myself and found this:

[Link: profshutdown.com]

which does seem to indicate that the packet consists of a hex value of 6 0xFF bytes. It also suggests that UDP can be used.

For those of you who are wondering, support of UDP via ProntoScript is scheduled as a part of the next firmware update. Release will likely be before end of year.