If you want to increase the number of processes running tasks to, say, 5 (by default, it is 3), pass the following:
npx nx build myapp --parallel=5
You can also set parallel based on the percentage of the number of logical CPUs.
npx nx build myapp --parallel=50%
Note, you can also change the default in nx.json, like this:
// nx.json
{
"parallel": 5
}