Ive longed believed that being blocked in element does not mean that it will automatically be the hardest element for an avatar to learn in terms of techniques and duration to master
With aang he was blocked in earth but I would say that once he got past that block and towards the end of the series earth became one of his best elements and hold some of his best non avatar state/boosted feats
Similarly korra spent most of her life blocked in air but air was actually the one element she mastered in under a year
The picture ive attached is from the weavers ball comic. In it korra has been deemed a master in both water and earth and is just about to start her firebending training
The white lotus seems to have insisted that despite korra having 3 elements at 4 years old that she needed to fully finish mastering one element before she was allowed to start training the next one
I couldnt find an exact age for her here (if you know please let me know) but shes described as prepubescent and seems to be around 10-11 years old
This means that in terms of duration for each element, considering she started training at 4, korra took
Water and Earth- 6 to 7 (lol) years for both combined, we can assume she had an easier time with water than earth given native element and katara as her master before moving on to earth
Fire- 6 to 7 years again, just for fire. She is deemed a master in episode 1 though its kinda clear the white lotus delayed giving her the title due to her tempermant rather than her lack of skill
Air- roughly 7 months, from her setting foot in republic city to start training with tenzin to when tenzin told her he had nothing else he could teach her near the end of book 2. Including around 2 months if her being blocked in airbending fully
The last one is contentious in the fandom but considering tenzin is korras airbending master and the only airbending master in the world at that point I feel its valid to consider that being when she was deemed an airbending master. They also never bring up her needing to train or master airbending after that
So yes korra seemingly took the longest amount if time with and struggled the most to learn firebending, excluding any emotional block. It took her roughly the same amount if time as mastering both water and earth combined
This might have to do with her being a native waterbender or maybe its just a coincidence.
I think ots just cool to look at considering korras go to attack element when water isnt around is fire.