I used free GPU-Z utility program to compare the so called display pipeline performance of Rhino 5 and Rhino 6. (I tumbled small drawing while making the following screenshot. The most important parts of the screenshot are highlighted with yellow color.) To my surprise, my Quadro K2000M graphics card worked twice as hard in Rhino 6 than in Rhino 5.
I opened the same small drawing in Rhino 5 and Rhino 6. Rhino 5 was faster in this test as well. Rhino 5 opened simple 3DM file in 42 seconds. Rhino 6 opened the same file in 55 seconds. I removed polygon mesh from the 3DM file and ran the test again. This time Rhino 6 opened the 3DM file much faster. Rhino 5 opened the file in 38 seconds. Rhino 6 opened the file in 16 seconds.
There are more problems with Rhino 6. It is buggy. It does not have the help file, only online documentation. Searching for a command in the online documentation is slow because search results produce lots of irrelevant links and because the list of all commands is not available. Rhino 6 does not have compact command prompt used by Rhino 5 for the Macintosh, but the old, wide screen hog.
Rhino 6 introduced new Tweedledum-Tweedledee commands: Parabola3Pt, ShowDir, and DevLoft.
My conclusion: Rhino 6 is not better than Rhino 5, just different.