I've been having this problem pretty much since launch. I mainly experience extreme frame rate slowdown when attempting to teleport from crowded areas like Revenant's Toll and Limsa Lominsa. Very rarely I'll make it to the zone I was teleporting to, but most of the time I either get stuck on a loading screen until I close the game manually or my client crashes.

Other times my frame rate drops when leaving Revenant's Toll for the other parts of Mor Dhona. If I go back the way I came I've noticed my frame rate returns to normal, but I usually have to restart my client in order to fix things. I also occasionally have weird problems in CT and CT2 where the game freezes almost to a halt as the boss dies, then returns to normal all at once after a few seconds. This usually results in a few (but not all) of the mobs in the areas up ahead becoming invisible as if the game can't load their models.

I'm using a Sapphire Radeon HD 7850 2GB video card and I've got 16GB of RAM. The game runs at anywhere from 40-80 FPS in most conditions on max settings. Because these problems only seem to occur after an hour or so of playing it does seem to me that it might be memory leak related, but it could also be something to do with my video card. I've heard a lot of users with Radeon cards report similar issues.