SpriteKit shader uniform update interval

I'm using uniforms to make smooth inshader animation. Because I can not obtain or change u_time I've use custom timestamp, animation start and animation duration uniforms, but, unfortunately, updating uniform values in update func didn't cause immediately uniform update(looks like it's updated only once per ~0.5 seconds, even when update called every 1/60 second). May be someone know how to get/update u_time uniform or update custom uniforms smoothly?