We will need to see Super Seven outputs plus enabled_blades to give a conclusive answer. S7PAC - Super Seven Performance Assessment Command
Even if you are using R81+ on your 6400, Dynamic Balancing/Split will not be enabled by default unless you manually enable GNAT because a 6400 has less than 8 cores. However even if you did that, I don't believe there is enough spare core utilization capacity for Dynamic Split to adjust from a 1/3 split to a 2/2 split to relieve the utilization on your single SND core, at least at the moment you took your screenshot.
Depending on Super Seven outputs there may be tuning that can be performed, which is all covered in detail in my new R81.20 Gateway Performance Optimization class.
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