Forza Apex is not at all similar to Forza Horizon 3's performance issues. Apex remains one of the best running titles be it DX12 in general or UWP specifically.If it's like Forza 6: Apex and this, it will. :/Good, I hope FM7 doesn't suffer the same fate.
A decently capable CPU + GPU will allow for 4K fully/nearly max settings at 60fps or greater with few drops apart from rain and certain conditions. Horizon 3 struggles to have "a few drops" only as there are portions where the framerate simply plummets 10-15fps or greater.
Granted the linear nature of Forza Apex means naturally it won't have the challenges of a more open environment streaming game like Horizon 3, but let's not act as though the two are plagued with the same performance problems.