The non high-speed goes to the other side of london and takes 30 minutes longer.
Sounds like I am years out of date but BITD the Faversham slow tickets were about 20% cheaper than a similar length journey I used to do down to Hampshire.
Victoria worked for me versus (IIRC) St Pancras but obviously not for everyone.
EDIT: I just looked this up and Faversham (fast) is about the same price as Petersfield for a weekday anytime ticket, whereas Faversham (slow) is 15% cheaper for a 10 minute longer journey than Petersfield. I think the suggestion is broadly correct.