Speed affects how fast your kinsect moves, power affects how much damage it does, and stamina affects how far it can go before it needs to return.
If you're just playing through the game, most people suggest that you just upgrade your kinsect's speed, since that's probably the most important stat as it allows you to gather the different extracts much more easily. Stamina is also pretty helpful. Once you get to the end of G-rank and can make rarity 9 or 10 weapons, however, this strategy becomes less effective, and at this point it's suggested that you make a new insect glaive from scratch, as there's no way to "reset" its kinsect.
As you upgrade your kinsect, you will notice that it gets different abilities depending on what you feed it, such as Ext Effic (S). If you upgrade your kinsect in certain ways, you will eventually be able to get one of four "final" kinsect types:
- Stored Extract +1: Your kinsect gets the ability to hold two extracts at once
- Pierce Up: Your kinsect will never bounce off of the monster (similar to the hunter ability Mind's Eye)
- Charge Time: If you hold down X+R, your kinsect will "charge", and when you release it, it will fly off faster and farther. This greatly reduces the time it takes to fully charge your kinsect
- Extract Extender: Extract buffs gained from the kinsect will last twice as long, including the red+orange+white buff
The vast majority of insect glaive players I see online use the Extract Extender bug. It's the best simply because you don't have to go gather extracts nearly as frequently as you would otherwise, meaning you can spend more time attacking the monster!
Charge Time is also worth a mention, especially the Blunt one, called Fleetflammer, as it has a pretty high motion value when fully charged and deals decent KO damage. If you like attacking with your bug a lot, this is the bug you want.
This video is a pretty good guide on how to upgrade your kinsect, and what the different numbers mean and which extracts to use, although it's for the Extract Extender kinsect and not the Charge Time one. It shouldn't be very difficult to figure out how to apply it to the Charge Time bug, though.
Finally, don't bother with element on kinsects. It doesn't increase their damage by much, and its only use is just giving your kinsect a special particle effect when it hits the monster or changing a small part of its color. (Fire kinsects get red, thunder ones get orange, etc)