Mod quality

By far the best farming method for mods is to purchase them from the store, making sure you only purchase those that meet your initial speed threshold. For most of us that means not buying mods that don't start with +5 speed, though if you're desperate for a particular combination (maybe pot set triangle with CD primary and also speed, off & pot secondaries, which would be more rare simply bc you're asking for so many things to come together) you're free to make that +4 speed (or even +3 if you feel lucky). 

You'll get enough free mods that it is probably worth using your mod energy to buy slicing materials. There are a couple slicing mats you'll run through fairly quickly. If you spend 150 crystals/day on mod energy, there's going to be more than enough to farm more mods, but I discourage this. 

Why? Bc refreshing the mod store costs 15c instead of 50c, and for each refresh you get more good mod candidates (and, after levelling and slicing, more good mods). Since you're getting more mods for spending 15c than you would spending 50c on mod energy, the latter is simply never worth it. Even on triple drop days it's very slightly less efficient. 

That said, constantly farming good mods from what's available in the store works over the long term, but you can run into a crunch where you just have to get more mods of a specific set bonus. In that case, some people will want to spend their mod energy on the mod challenges for that set type. You do you. Just know that's not the most efficient use of your limited resources. 

When I'm desperate for something, instead of trying to farm with mod energy, I tend to just buy green, blue, or purple mods that are almost perfect, but are lacking one secondary stat that I want for that mod. You buy it, you level it to show all stats, it probably doesn't have that last stat you want and so you sell it. But you did give yourself one more chance, and over time those chances will eventually come through for you. 

That's enough for this post. I'll also link another thread on mods in a reply post to this one, one I wrote a couple months ago.