Depends on the operation in question. Direct syscalls should be faster, but there are some big differences between the NT and Linux kernels. You end up getting all the poor performance areas of BOTH. That, and HyperV has an extremely fast lightweight VM mode for particular hardware. For example, running containers on HyperV actually runs them with VM level isolation, a la Kata containers... and at competitive speeds.
An example of the worst of both worlds result is the filesystem. In the NT kernel, every filesystem call goes through a series of Filters: any program can provide filters for given file types. Filters can do anything, from antivirus programs scanning the file on access, to Notepad adding itself to the Context Menu as an "open with Notepad" item. This makes filesystem access - even metadata checks - very time expensive on Windows. On Linux, we keep file metadata in memory, which is memory expensive, but it makes certain filesystem operations very fast. The result of mapping one onto the other is a combined filesystem operation which is memory expensive AND slow.
So for example, on WSL 1 (mapped syscalls), using a git repo of any size is impossibly slow... on the order of 10 seconds to get a git status for a work project. But on WSL2, the same syscalls live entirely inside the VM and are almost native fast.