Frustration
Frustration
[Black bra with Rubik's cube closure.]

A bra is pictured here, but instead of a traditional clasp, a Rubik's cube is used instead. The comic is a metaphor for how many men (and potentially women who don't wear bras) have difficulty removing their woman lover's bra before sex.
Women who wear bras are going to be quite adept at unhooking their bras simply due to experience and muscle memory. However, many common bra clasps are unique to bras and aren't found on other clothing, so their bra-less lover, lacking that experience, might fumble with the clasp for quite some time. In the heat of the moment, a lover may feel frustrated if they feel that the act of removing a bra is needlessly complicated and distracts from the activity at hand (namely sex). Depending on the clasp mechanism, attempting to remove the bra may seem as frustrating (hence the comic title) and complicated as solving a Rubik's cube.
A literal bra clasped with a Rubik's cube would doubtless be especially frustrating, as someone wishing to remove the bra would have to solve the Rubik's cube every time in order to undo the clasp. This would doubtless prove annoying, especially for a partner who may want to remove the wearer's clothing without impediment.  A normal bra clasp can be tricky enough for the inexperienced user, but the addition of the Rubik's Cube element is going too far. However, it is possible that this is actually a nerdsnipe bra, as it appears to be a front-clasped bra, making the Rubik's cube a red herring.
The title text is an imagined conversation between someone trying to undo this bra and someone who is likely wearing the bra. The first person explains that they can "do it" (i.e. undo the bra) in under a minute, which is a reasonably impressive skill to have if you are not a professional speedcuber . The second person replies that they've noticed, a sarcastic reply that relies on the alternative, sexual meaning of "do it," implying a complaint about the first person's speedy performance in bed.
[Black bra with Rubik's cube closure.]
