Subsampling makes sense only with PT+PT. Obviously if you use PT + UHD, there will never be fluid feedback, because UHD need to precompute. There's plan to make actual UHD precomputation usable for subsampled scene preview, but that won't make it in 1.6. Although, we will probably get a check box to always force PT+PT for IR.
If you use PT+UHD in IR even in super simple scenes, your feedback will be terrible.