I preformed a lot of testing on the game with regards to performance(worked in development before and researching for myself possible solutions for a game) and this is what I notice. To start I want to say I loved the demo and a huge fan of the game design. The problem is once the ball is thrown and you see the portal, the game take a huge hit in performance. I didn’t notice this initially however when bumping SS to 1.5, I immediately noticed reprojection. Looking at the timings, it looks like there's about 60% more frame latency when having a portal open, most likely due to it having to render the scene twice. This is not an easy problem to solve however it is extremely important. One possibility, is to limit the resolution of the portal and have a blur effect on it. This may reduce the utilization and slightly adjust the gameplay as it will raise the difficulty a bit. Please take a look at this, as it makes games incompatible with SS.
i7 5960x 16core 3.5ghz
The processor is definitely not the bottleneck as its got 40 pci lanes and is on an x16 slot. If you have more specific testing I’d love to be able to help.