Generalizing from research, we can predict that the presence of others will performance on well-learned tasks and performance on novel or challenging tasks.

Generalizing from research, we can predict that the presence of others will INCREASE performance on well-learned tasks and DECREASE performance on novel or challenging tasks.

in doing we're doing a well-learned task, our brain could complete it with less focus, so having the presence of others will give no trouble to our task.
But, in challenging tasks, our brain need more focus to complete it, so the presence of others will be more likely to cause nervousness that lead to a decrease in performance