my ping shows around 32ms - at any time of day/night, then i spent an hour on the phone with 2nd line support at my isp looking through tracerts together - where they said the issue is with square enix.
secondly i tracert the server ip (195.82.50.52) at different times of the day - and i noticed and verified this with 4 other people who live across eu/uk with different isps that the ip (195.82.50.234) which is just before the final ip has a crazy ms range

11 am gmt - 370ms
1 pm gmt -550ms
7pm gmt -1332ms.........
1am gmt - 32ms .........

As you can see at 7pm the game becomes totally unplayable for me - even using wtfast during these times it reports a 500 + ping (still unplayable) however when i test it at 1am wtfast shows 32ms ping - mmmm that buttery smooth game play : )
and i can even play without wtfast but i get a bit more overhead from my isp. What i don't understand is why some like myself seem to be affected at peek times while others are not.