*Note - I did this on PC simply because that is what the guy did in the video. I do not know if or how well it will work on console.
If you're one of the unfortunate people to suffer from the glitch mentioned in the above solution where the monkey doesn't spawn, or accidentally picked up a Black Thistle, this is the absolute fastest way to play through the game:
There's only one thing you really need to do to complete this game. Collect all four "Mother's Wish" paintings. Following this video will do just that in the most efficient way possible, and not monkey around (pun intended) with the side stuff.
Generally, following a speedrun video as a walkthrough is a terrible idea due to glitches and shortcuts that take a very long time to master. However, this guy's glitches can be easily performed by just simply mimicking the video. (just be especially careful when he's going up the first mountain to take the picture that goes in the 3rd slot.)
It took me about three and a half hours to follow this 10 minute video, but if you read and follow these tips from the start I am sure you can do it in under or a little above an hour.
*BE SURE TO GET THE MAP FROM THE KESTREL'S AERIE!!!!*
I did not do this because I didn't see him collect the map in the video. It is IMPOSSIBLE to do this without the map, as you'll get stuck in the island to the right of Lyndow. Get the map on your way to the fourth painting. (talk to the guy upstairs and it will cost four Inky caps you'll have to find.)
*He performs a glitch where he only uses two canvases to complete the game. I have not figured out how he does this, so you will need to look out for boards and fabric whilst on your adventure. There should be enough if you combine what's in Nava and what you can get when travelling to retrieve the map.*
Also.. Do not pick up any of the Black Thistle plant that he does. You'll likely be short on cash after the selling spree anyway, (Because you do need the bike..) Instead, complete the quest that allows you to do art commissions, and do the one that has you take a picture of a depressed person. The person who you need to take a picture of is in Nava and nearby wearing a blue shirt.
If you follow this, I cannot really foresee it taking longer than an hour, but definitely not longer than a mainline quest runthrough.
This is not an achievement guide. If you have not gotten all of the miscellaneous achievements it is highly recommended to follow the walkthrough on TA.