OneLoop may return incorrect results when used with the t'Hooft-Veltman scheme, hence the message. Please use TID instead, it supports all kinds of output that one might need from OneLoop.
As to parallelization, there is no native support for that in FeynCalc.
Usually, if things get too slow, you need to consider switching to FORM, especially when it is about computing matrix elements squared with complicated Dirac and color traces. This is what I do myself.
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