You need to tell Gurobi to use more threads:
using JuMP, Gurobi
model = Model(Gurobi.Optimizer)
set_optimizer_attribute(model, "Threads", 12)
Try 12 threads to max out your CPU, or a lower value to reserve some capacity for your computer to do other things. Don’t expect much benefit from going higher than 6 (the number of cores in your CPU).