Really hope Squareenix'll take a look at this; it exactly describes the behavior I've been experiencing from the launch of Stormblood. Thought I'd pitch in a few things I've observed as well!

The several-second freezes appear to be happening when the game is struggling/failing to load a model/texture. I first noticed this happen in The Peaks where I was greeted by a completely invisible enemy near Hidden Tear shortly after experiencing freezing behavior. On another occasion, I experienced freezing right before the final boss in The Sirensong Sea, and as a result the boss was invisible to me the entire fight.

The client itself usually crashes with an unexpected error less than an hour after freezing behavior begins. My AMD drivers are completely up to date, and I use a Radeon RX 480 graphics card.