As a rule of thumb, with not huge numbers but a few very small batches of different native / traditional and mixed types, my thoughts are :
Grower pellets designed for trad breeds from weaning to 4 months, loosely following the quantity guides in the article here that Dan linked, but
never going over 4lbs per pig per day for a trad type unless it is clear they need it. From 4 months to send off, I usually switch to sow rolls but I take harmony's point : it depends on what you are trying to produce. I always prefer something that isn't pushed and am happy for them to eat less / lower protein for longer, but that won't suit everyone.
The lady who used to look after the pigs here had it down to a fine art. Start them at 8 weeks on 1.5lbs per pig per day in two feeds, up the feed a little as the pigs were clearly wanting more and more ready for their meals, watching their condition so you can always
just see the line of the base of the ribs on the flank. Max 4lbs per pig per day, but actually that is often not reached here as we have a lot of veg waste from the veg plot and also lots of whey and other dairy waste from the dairy. Being guided by their condition and hunger levels means the system automatically flexes and produces similar finished pigs no matter what the breeding. (With the exception that the shape of the hams and the size of the eye of the loin will be more rounded in less trad breeds in general, although our current supplier produces very shapely Cornish Lop crossbreds - no continental blood in the mix.)
We switched from the local agri merchant's pig feed a few years ago, when we discovered that they had quietly started including GM soya.
Since then we have used Smallholder Range (non-GM non-soya all UK-grown.) The pigs definitely grow less quickly and remain more lean on this diet, but we are happy with the final product we get.
ETA One other point. My observation is that a Pietrain has very shapely hams from the get-go, pretty much, whereas the Cornish Lops and other trad breeds do not develop shapely hams until later on. Therefore, if you accelerate the pace for a trad pig, you may find it counterproductive, in that you finish it at 6 months, but that is actually at a suboptimal conformation, and if you finished it for 8 months you might have better hams and loins. Just a thought