Bob Mueller was completely lost, confused, and incoherent at times. He repeatedly asked that questions be repeated.
Watch this video to see the important clips of Mueller's testimony. It cuts to the chase.
"Mueller looked like a confused, stuttering, mumbling, lost, doddering and nervous old man. He frequently paged through his notes."
Even though Mueller leafed through the pages of his report to answer the questions at hand, Mueller could not find the answers to the questions. Obviously, Mueller did not write this report as he was not familiar with his own report. So two questions were asked: (1) who wrote Mueller's report, and (2) who wrote Mueller's ten minute speech that followed a few weeks after this report was delivered to Congress? When asked these two questions, Mueller refused to answer.
Even Chris Wallace remarked that Mueller's testimony was a disaster for the Democrats.
Even though there may be areas in Mueller's testimony where perjury could have been committed, due to Mueller's frailty real or faked, I doubt that any charges of perjury will stick.