Practice Tips

Practice all motions for accuracy, not speed. It should feel "easy" to perform every motion.
Upon every failure, take note of why that input failed. Did you rush the button press? Miss a diagonal? Adjust your technique or timing to be more consistent.
Your speed will naturally increase until it is fast enough that it works in-game, because the motions become too easy, not because you are rushing. For combos, and cancels from normals, this is as fast as you will ever need to be.
Always operating at <50% of your capacity is what gives you the headroom to pull out clutch plays and adapt to new situtations.
You can solo, mute, or boost priority of moves in the queue using the 'Focus' button to the left of the move.

Input Devices

One of the original motivations for making this site was to be able to benchmark and compare devices, but I've since realized that as long as a device has consistent behavior they are all possible to get consistent inputs with. It's all about you adapting to the controller. The main difference between devices is do you like using it. So modding a controller is less about making it perform "better" and more about just making it something you like.
D-pads and levers have four physical switches that represent the four cardinal directions, to activate a diagonal means triggering two switches. This means you have to physically push 41% further into the corners to get diagonals than you do a cardinal direction. All of the 'circle' motions are actually square.
The main difference between American, Korean and Japanese levers is how do want to control a stick. If you want to use your wrist and forearm, consider a Korean or American lever. If you want to use your wrist and fingers, a Japanese lever might suit you better.
A leverless controller will be slower than a Stick or D-pad unless you invest in learning SOCD shortcuts. You can benchmark this yourself using this website but it also seems to be true looking at pro player replays.