Envs seem not to work in parallel

[AgentEXPL]

Only one Data worker is working since config.num_actors=1. Inside one Data worker, there are several working envs. But they works in in serial rather than in parallel, as follows.
for i in range(env_nums): obs, ori_reward, done, info = env.step(action).
Is it possible to speed up data collection by setting config.num_actors > 1?