In some models, the continuous root relaxation takes significant solution time before parallel branching begins. These models have potential for additional speed increases by running the root relaxation step in parallel. If the root problem is an LP or QP and the Threads parameter is set to a value greater than 1 (one), the concurrent optimizer is invoked by default. This provides a form of parallelism that assigns the available threads to multiple optimizers. If the root problem is a QCP, the barrier optimizer alone is used.

You can try a different form of parallelism at the root by selecting the barrier optimizer specifically with the starting algorithm parameter:

In such a case, the parallel threads will all be applied to the barrier algorithm.