The refresh rate in PV and Editor is not real-time because at that place Cinema (except of Editor) requrests all AOVs and the main image too. Getting and saving them to Cinema's PV is quite costly (especially for many AOVs). So refresh rate was decreased.
IPR has much cheaper picture delivery (and updates only what it sees).
Also the IPR has Final Geo mode (FIN GEO button) which renders the same scene as PV (Final render) with all tha final subdivs etc. In this mode it doesn't respond to geometry changes but it reacts to material changes which is a cool thing. Then you just send it to PV with the most left button in IPR and continue working there.
When light tags will relocate to materials there will be also possible to change lighting in PV-like mode (as described here)

Bucket mode in IPR can't be switched off. If you mean that that it starts at some left-bottom corner and reaches the desired place of image at some longer time - yes it's a problem. At least the region render can be used to guide the bucket. And maybe later I will make the buckets to restart from the most usable areas.

Some noisy stage before buckets is used to preview quickly and populate the caches silently. But the quality of noise will be improved and the stage can become shorter (to avoid wasting time).