Obviously the fastest time is going to come from building up candies and investing them in Buffed Out + for your record attempt. There are two ways to acquire candies here, candies from shards and candies from stopwatch mode. After getting Buffed Out Infinity this was my strategy.

For shards I took the Money Multiplier up to an X8 with 13 candies and then split the remaining with 2 candies going into Rift Multiplier for every 1 going into Buffed Out +. I would then play until I hit the Rift limit and got the last candy I was working on if it was close. I would focus on Elastic Cables, Pickpocket, and Cake Eater, while keeping Rocket Slam, Luck, Doors Destroyer, and Aerodynamics at about half of those. I did this until I could afford to bring Buffed Out + up to 50 while keeping Money Multiplier at X8. This requires 62 candies.

After bringing Buffed Out + to 50 with a Multiplier of X8, I played through rifts until I could max out Elastic Cables, Luck, and Aerodynamics, then played stopwatch mode until I got to Level 25. This brings you to around 85 candies. From here you can continue building your candy count with either method to improve your final time.

Now allocate everything to Buffed Out + while leaving Money Multiplier as low as you are willing to and you are ready to go for the record. I kept Money Multiplier at X8

For the 2nd Level speedrun avoid a critical hit or a KO as the animation for these takes about a second and is slower than bouncing off the opponent even though you get less speed. With Buffed Out + above 60 this is easily a sub 1 second run. At Buffed Out + 100 you can get below 0.5 seconds. I believe a higher Buffed Out + Elastic Cable is the only way to make this faster.

For the 6th Level speedrun the initial strategy I used was to wait for Jon Zzzeena and attempt to clip the bottom of them. There is a near frame perfect trick where you can hit just under them and grab the speed boost without triggering the full crit animation and being launched toward the sky. From my testing, the closer Jon Zzzeena is to your character when you launch the less time is spent on the crit animation and the faster your run will be. Try to avoid getting caught in the clouds as the animation for hitting the ground in a cloud causes the game to run in slow motion and wastes about a second. Hitting the clouds at speeds over 4000 also can cause the game to crash. At Buffed Out + 60 with this strategy you can achieve runs in the 10-11 second range. Unfortunately if you go above Buffed Out + 60 it becomes difficult to complete runs. The game will either freeze or you will be moving too fast and you clip through the final wall without breaking it and triggering the ending. At Buffed Out + 100 I did manage runs that reached the end in less than 7 seconds but never actually broke the final wall triggering the end sequence. 

This means that the best way to lower the time from the 10 second range is to build up your Buffed Out + and again avoid hitting a Crit when you strike the opponent. You get less speed which prevents the game from crashing and you save about a second since you don’t trigger the Crit animation. You again save a slight amount of time if you hit your opponent when they are closest to you which makes Villaina Ice the best to use since they are the opponent that can get the furthest to the left side of the boxing ring. From there dropping bombs from Cakesplosion inside each of the Giant Doors will increase your speed throughout the run. This strategy starts being faster than the John Zzzeena trick at around Buffed Out + 80.

Both of my records were done with a Buffed Out + 100 character with everything maxed except for Bounciness and Bodybuilding which I left at 0 to avoid hitting the clouds and give myself more attempts before I eliminated the opponent. I also left Luck at 30 in an attempt to reduce objects on the screen and make the game more stable but I don’t think that this actually had any impact on game stability.