It is supposed that changing the real-time attributes of a process it will be faster
This is definitely wrong.
Scheduling whatever under a real time policy might help achieving precise & deterministic latency, sync between tasks… but since there is no free lunch to the expense of throughput.
In the particular case of whatever cpu-bound task (what a video encoding task is) what you want is : throughput !
Therefore… scheduling that sort of task SCHED_RR will only kill the performances of everything on your system (this including… your benchmarking application).
And the more your cpu-bound task is multithreaded (likely with modern cpu-bound tasks), the more degraded perfs you will observe.
Renicing it should be just far enough to achieve better performances or, at the extreme limit, if highest possible performance needed then pin this task on some idle cpu.
AND, BTW : Never never and never schedule anything RT with the max possible priority. If anything goes wrong, you'll never be able to do anything else but a hardware reset.
Not to say BTW 2 : Benchmarking whatever RT system with whatever benchmarking application running under that same system is just : absurd.