Mario Kart 8 Deluxes Booster Course Pass allows you to cut as many corners as you can in order to stay ahead of the pack. Secret paths and quick diversions off the track are no strangers to providing quick cuts that will save you a few seconds.
Golden Dash Cup shortcuts
The Arc de Triomphe is topped off by a couple of cardboard Piranha Plants, which you can avoid on your first run, but when you come back, you can use a Mushroom to blast through them without slowing down or taking the loop. Right before the finish, you can repeat this route.
Toad Circuit is a fairly standard race track, with little to no downtime. Just outside that, there is a small sandy area with pipes. If you have another Mushroom, go through here without hitting the pipes.
Except for making minor adjustments, there are no real shortcuts on Choco Mountain, such as in the cave tunnel with a Mushroom.
A shop you can use a Mushroom to go through past the first set of escalators on the right is a store that you can use a Mushroom to go through. If you don't, the floor will slow you down.
Lucky Cat Cup shortcuts
On the first two laps of the track, you can hit a Mushroom ramp.
Shroom Ridge has two shortcuts. The first is a grass ramp that you can hit with a Mushroom once you reach the grass area. If you use your Mushroom too early, you will be slowed down right before hitting the ramp.
The second shortcut is at the end of the winding road. Where the barrier is missing, you can use a Mushroom to blast onto a wind current that will propel you back onto the road.
Two leaves you can drift into and do tricks off of around the first turn in the lap.
After that, you can use mushrooms to help you through some grassy areas, and when you see a beanstalk poking through the course, you can use a Mushroom to cut off the twisting turn there.
There are a number of approaches to Ninja Hideaway, but none are much quicker than others. There are also a few cardboard Piranha Plants you may smash through with a Mushroom in the rooms with the spike ceilings that rise and fall.
Turnip Cup shortcuts
On Mario Circuit 3, there is a break in the block wall that you can easily pass through with a Mushroom. You may also shorten your time by using Mushrooms around the track's curves.
Depending on the lap you are on, the Kalimari Desert has a few ramps you may hit. The first one is right after the starting line, but you will only have access to it on the first and second laps.
If you have a Mushroom, you can hit the second ramp right after the first railroad crossing.
The last ramp is just before the finish line and is only accessible on the first and final lap.
Propeller Cup shortcuts
The second lap of the Sydney Sprint will allow you to use a Mushroom to cut through some grassy areas. Be careful here if you hit any rocks, as this will slow you down exponentially.
The second one is a simple cut through the grass just before the finish line. Technically, this is available right on the first lap as well, but with it being at the start before any item boxes, you will just slow yourself down.
There are plenty of spots in Snow Land where you can save a few seconds by using a Mushroom to cut through some snow around corners. The most obvious one is just before the last turn to the finish line where you can speed through a crack in the wall.
A Mushroom ramp just after the wall section is available.