"When congressional staffers, prompted by repeated media inquiries, asked Mueller’s team about his cognitive acuity, they were told — three separate times — that he was okay...."

"After Mueller’s halting, sometimes confused testimony before two congressional committees Wednesday, some lawmakers are privately wondering whether there was some truth to the rumors — and whether they were right to force him to testify against his wishes....  Democrats lionized Mueller, believing his investigation to be their best hope at exposing wrongdoing by Trump.... After Mueller’s investigation concluded — and Democrats pressed him to testify — his staff communicated to Capitol Hill in no uncertain terms: Mueller did not want to do it.... For a time, Mueller’s team pushed for the hearings to take place behind closed doors, and they advocated aggressively to limit each of the hearings to two hours. Members also were perplexed that panel staffers wanted them to shape questions so they could be answered with a simple 'yes' or 'no.'...  Some Democrats recognized after the first hearing that Mueller was not as sharp as they would have liked. During a break, Rep. Eric Swalwell (D-Calif.), who sits on both committees, warned lawmakers on the second panel to slow down, shorten their questions and speak louder so Mueller could follow better.... Democratic lawmakers are divided about whether they made the right decision in forcing Mueller to appear. Some — and most committee staff members — say they had no choice. While saddened by the attacks on Mueller post-hearing, they say it will pay off in the long run for their investigations, as the public event allowed Mueller to publicly confirm unflattering facts about Trump that they can further explore."

From "Mueller’s team told Congress his acuity was not an issue. Some lawmakers privately worry it was" (WaPo)(relying on anonymous sources).