Oreo Colorizer is an app that can force your device to shift from using the standard color space to a wide color gamut. On the Pixel 2 and 2 XL, this means colors will no longer display at just sRGB, but instead with the DCI-P3 color space. This works in all apps, as well as on the home screen.
Optimized Rendering only affects apps that use SurfaceFlinger (ie: 99% of all Android Apps). This mode has ZERO performance penalty.
Any app that uses special drawing techniques (SurfaceViewer) will require per-frame rendering at the cost of performance. I suggest only using it only for special cases (like watching video).
Read more about Oreo color spaces: http://www.androidpolice.com/2017/10/25/oreos-color-profiles-work-pixel-2-2-xl/