single word requests - Name for “having no concern about the truthfulness of one’s own remarks”

Take the following statement:

I hate the ease with which X makes disparaging remarks about their rivals without the least concern as to whether they are truthful or fair.

Now I would like a word expressing this lack of concern, so that I could just plug it in the blank below and keep the meaning of the sentence above:

I hate the _________ with which X makes disparaging remarks about their rivals.

This is an all too common attitude and I think it deserves a name of its own. The word that came to my mind was levity, but I haven’t come across this word very often and I’m not sure of its particular tones and shades of meaning. Merriam-Webster defines levity as:

Simple definition
: a lack of seriousness
: an amusing quality
Full definition
1 : excessive or unseemly frivolity
2 : lack of steadiness: changeableness

The attitude I’m trying to name certainly displays lack of seriousness as to the truthfulness and fairness of the remarks, but I do not want to imply they are made in jest or that X is being silly or thoughtless. X could be very serious about their aims: “I’m going to say this; don’t give a dam whether that’s true or not; even if it’s not true I’ll get away with it.” This excludes, I think, all the words Merriam-Webster lists as related to levity, which probably means it excludes levity too:

facetiousness, flightiness, flippancy, frivolousness, frothiness, frivolity, light-headedness, light-mindedness, lightness, silliness

So I’ll accept an answer that despite my reservations shows that one of these words is indeed the one that works best in the sentence above; or gives a more fitting word; or shows that no word really fits (I am of course aware of the difficulties of proving a negative).

I prefer a single word but I’m willing to trade singleness for meaning: so I would take a short phrase it fits better than any single word.

Update

I’m spoilt for choice. But I wonder whether the various suggestions implying lack of concern would in my sentence imply lack of concern specifically for truth, which is what I’m after, or could instead be interpreted as lack of regard for norms of polite behaviour, rival’s feelings, or consequences (say, libel lawsuits, rivals reacting with violence, people feeling encouraged to attack the rivals, etc.)

Tweak the example sentence if it helps (say, I hate the ____ of X’s remarks).

Is there a word/phrase for "unperformant"?

As a software engineer, I need to sometimes describe a piece of code as something that lacks performance or was not written with performance in mind. Example: This kind of coding style leads to unmaintainable and unperformant code. Based on my Google searches, this isn't a real word. What is the correct way to describe this? EDIT My usage of "performance" here is in regard to speed and efficiency. For example, the better the performance of code the faster the application runs. My question and example target the negative definition, which is in reference to preventing inefficient coding practices. Answer This kind of coding style leads to unmaintainable and unperformant code. In my opinion, reads more easily as: This coding style leads to unmaintainable and poorly performing code. The key to well-written documentation and reports lies in ease of understanding. Adding poorly understood words such as performant decreases that ease. In addressing the use of such a poorly ...

Notes

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