Threaded cognition theory predicts that switching is opportunistic and depends on availability of cognitive resources. Laboratory studies of multitasking suggest people are rational in their switch choices regarding multitasking, while observational studies suggest they are not. To establish whether effective multitasking can become ineffective we introduced delays in the primary task. The participants answered emails by looking up information (similar to customer-service employees) while being interrupted by chat messages. When participants were faced with a delay in the email task, they switched more often to the chat task on high-workload points. Choosing to switch to the secondary task instead of waiting made them slower. It also made them forget the information of the e-mail task half of the time, which slowed them down even more. We concluded that people’s rationality in multitasking behavior is only local, which agrees with the threaded cognition account of switching.