So many factors are not being taken into account. Your ISP will have multiple POPs (Points of Presence) if the closest POP for your ISP is in JHB and you're in CPT, guess what, your connection will suck. Then your ISP will have IPC (IP Connect) links for international connectivity, this will generally break out onto WACS, SAT3 or EASSy. Likewise being in JHB is a distinct disadvantage (with the exception being WACS), however the difference here is less noticeable than being far away from a POP. Also keep in mind that your traffic has to travel though the Telscum network to the closest IPC point to be transferred to the backhaul carrier for the ISP, then to the closest POP for the ISP, then from the ISP's POP to the next IPC to break out to an external network/international link, etc etc ect.
TLDR : While some ISPs might suck for some users, it will be awesome for others.