A developer has created an app that can test the 60fps rendering capabilities of the S4 SIP, the processing platform found in the 4th-gen Apple Watch. He has reported that the watch equals the iPhone 6S in this test. The watch also used less than 70% of its processing power while running this app.
https://www.notebookcheck.net/Apple-Watch-4-found-to-be-just-as-fast-as-iPhone-6S-but-only-for-certain-metrics.337036.0.html
I believe there is some misleading here, since ppi is not a unit of screen resolution, but of resolution density. Therefore, the S4 still has less pixels to drive, compared to the iPhone 6s, making it easier for the watch. Still impressive, though.