Hardware tone mapping is not yet available for QNAP.
Adding it would require compiling the Beignet and OpenCL libraries on QNAP / in their QDK for X86_64
Software tone mapping is available and it does use a fair amount of CPU.
On an i7-7700, I can manage one, maybe two, sessions.
The Linuxserver.io Docker images do contain the binaries which support hardware transcoding.