He underperformed because the debate wasn’t in Spanish....his mother tongue it seems. Of course he’d be wooden, halting or less certain. English as a second language is difficult to master.
The illegal immigrants Jeb Bush frolicks around with (such as his wife) can’t even speak Spanish well, much less English. Humans speak like their associates, and Jeb’s associates can’t speak a language at all.
In lacking linguistic capabilities, Jeb cannot construct a sentence in any language without resorting to artificial “compassionate conservatism” (AKA closet liberalism) platitudes. If you ask the man to speak about his non-political hobbies, he would stumble over his words more than a first-grader would.