Is doubting a claim more taxing in terms of energy spent by the brain? I was at a conference some time ago in a university near my city, a Neurologist was part of the various talks held there, and in his words: 

"There is a, let's call it Neuro-Psychologic interpretation of belief, that is actually supported by evidence, which in essence says we really, really don't like doubting what we believe. 
Because not only does it take slightly more energy to doubt any given claim, but by default we are believers. We believe things said to us, and then subject them to scrutiny in a second moment; this according to research conducted in Harvard using an fMRI. Now, if we add to this- in Piagetian terms which most of you are familiar with- the notion of a Schema, so, linked thoughts and expectations placed on the world to simplify actions and problem solving, we are in big trouble. Daniel Kahneman, a known researcher and Psychologist has basically set in stone the fact that we are lazy thinkers. We like to take shortcuts, we don't really like hard challenges and Confirmation Bias would rather us keep believing things we already do.

If we were to pick between rational discourse with the opposite side, or lash out or ignore them, we'd pick any options that would let us avoid confrontation, because it's really exhausting for the brain to take head on, a waterfall of information that disagrees with more or less your whole worldview, it's too much energy. So true skeptics who enjoy doubting are anomalous, lying or masochists if you ask me."

I translated this in English because it isn't the original language of the talk.
Do his claims have any weight? It would be a stroke of luck if there was some researcher from Harvard to confirm what he said about the fMRI case. He mentioned Daniel Gilbert was part of it, but i can't find the actual published paper for some reason.
Overall, the claim seems to be that people prefer holding their current beliefs. This is well-documented, for example in a big literature on https://en.wikipedia.org/wiki/Cognitive_dissonance . Note that this may be culturally dependent; some Buddhist traditions explicitly value holding contradictory beliefs.

It's possible the study being referred to in the quoted passage is reported in this paper: https://journals.sagepub.com/doi/10.1111/j.1467-9280.2008.02226.x 'When the Truth Is Not Too Hard to Handle: An Event-Related Potential Study on the Pragmatics of Negation' (One author is at Harvard).

If so, the quoted speaker's comments are inconsistent with the findings. There was earlier work suggesting statements with negations might be more difficult to understand, but this paper doesn't find that. Their results "...suggest that negation poses no principled obstacle for readers to immediately relate incoming words to what they hold to be true."

I think it's fair to say that humans limit the mental effort they put in; this perspective is also summarized in the theory: https://en.wikipedia.org/wiki/Cognitive_miser

A word of caution: the quoted text seems to rely heavily on vague appeals to authority (e.g., Harvard) rather than references to specific empirical findings. This can be a red flag for argument quality.