Does UE4 need to be installed on the bootdisk, the same drive Windows 10 in installed on, for best performance? I know the application being used and the file being operated on need to be on the fastest drive for best performance, but I am not sure how important being on the boot disk is.
no, best to get his via the launcher, if your looking into using the git version. follow this. Building Unreal Engine from Source | Unreal Engine Documentation
It is just a performance question. Is UE4 faster when installed on bootdisk, where Windows 10 is located, assuming both drives are the same?
No. no it is not.