Not always a problem at the end points (your client or SE's server). A very large portion of our issues are due in part (if not entirely) to issues with the routing your ISP's have concocted.

Run some traces to your game server (can find it by running resmon from your run/search box and watching what ffxiv connects to when you are logged fully into the game) as well as some other common sites like google, netflix, yahoo, TwitchTV, etc. to see if you spot highly erratic response times (or intermittent timeouts) along your routes. If you see them, forward the reports to your ISP's Tier3 support techs--they are the ones that know what to look for based on what shows up in those tracerts, and who to contact to fix your routing.

Your ISP has the service agreements with the people responsible for such third party segments where we keep seeing the problems (will show up with other ISP's domain names like level3, cogent, i-web, telia, TATA's as6453). SE is basically powerless to do anything about problems that occur outside of their internal network or on Ormuco or JP-NIC segments. At best, they can try to contact those 3rd parties on your behalf, but they don't have any enforceable service agreements with them... but your ISP does.