Tune encoder for encode dynamic video scenes with cbr option enabled

Please try to configure virtual buffer size:

        -vbs <size>           Virtual buffer size [Default = 0]

Here is a reference link:

And hardware preset type:

        -hpt <type>           HW preset type (1 = ultrafast, 2 = fast, 3 = medium,  4 = slow)

Please note hardware preset type improves image quality but affects encoding speed. Slow type may not be able to achieve 4Kp30.