I have started using Fast Raw Viewer and find it fast and really effective. Also pretty inexpensive. It displays a histogram based on the actual Raw data (and not the embedded jpg) which makes it great if you use a technique like ETTR. I can cull through all my Raw files quickly, and not wait 5-10 seconds for each image to display - it's as close to instantaneous as I can imagine. Link: https://www.fastrawviewer.com/