For GPU renders the bucket size is important to speed. And, it's super subjective. Scene complexity, system performance, memory and bus speeds. The only way to know whats going to perform best it to do speed comparisons for your self. Fortunately Poser has some tools to help you objectively tune your bucket size. If you look at the Message Log in Poser (the speech bubble icon on the top right of the screen) it will show you exactly how long a render took to run. This will give you the data to fine tune bucket sizes to your system's performance. On my RTX 2070 buckets around 128-256 seem to perform best. Also, adaptive sampling works best with progressive render off.