Samsung uses monolithic custom kernel for their android flavour
But issue of performance with exynos chip is not because of kernel .. vendor images for kernel drivers are closed sources for samsung ( google , apple , panasonic etc they all have some side of closed source )
Main issue of gaming with exynos chip is because of the nature of games or app developers development process .
Many will say this Exynos chip cores speed etc etc but . Just use strombreaker kernel v 5.x than we can easily witness how fast and responsive exynos become while playing heavy games
There are many examples ..
List and technical details is too long
But to get adapted with Samsung exynos based kernel environment games and apps needs more and more exposure and test over it
Hope next exynos wirh rDNA codes will bring universal platform into CS pf kernels too