You're probably noticing the scaling formula as you level. 1500 skill speed at level 71 won't give you the same amount of GCD reduction as 1500 skill speed at 70. You'll need to keep adding more skill speed as you level, possibly with materia if you don't get the right drops. It will probably stay wonky until you reach level 80 and can fine-tune the stat to fit perfectly with your rotation and regional ping.
There might also be some delay with the heavy server congestion?