Graham,
Ian is spot on regarding the tool unless of course you are intending to use the tool upside down and run your lathe in reverse [take care if its a screw on chuck] and cut outwards from an undercut in which case you could use a fairly high speed because you have unlimited run out
Another point are you intending to use full form or partial form inserts, if you use partial form then you need to truncate the thread crest, for 5/8 UNF this .006" / .008"
As far as speed is concerned I would not worry too much I have cut 1000's of threads and mostly at under 100rpm and never a problem with finish or otherwise. Yes I know the cutting speed for carbide is much higher but so it is for HSS but you have to be practical and a CNC machine is capable of stopping and retracting the tool in a fraction of a second most of us humans are not and if screwing up to a shoulder or undercut you have to be able to stop your tool before a crash occurs.
John