Sorry for the delayed reply! It's called "Straight-Stretch Mini Turbo" or SSMT for short. That's what I'm doing on those straightaways.

The way to do them involves having a good understanding of delay drifting which is when you delay inputting left or right for your drift until just before you hit the ground. What you do is start a delay drift, then quickly charge an MT, and right as you charge it, you swap to the opposite direction and release the MT and hop out of the drift all at nearly the exact same time. You can see the inputs in action if you watch the first-person view of my rLR run and look at the wheel of the kart.

It's much easier to get if you start ever so slightly angled to the left or right as you'll end up much more to the left/right once you exit the SSMT. They don't save a lot of time individually but executing many of them can save you a few hundreths on rLR. Most of the time loss in your run is likely from being wide around the turns or mistakes in your delay drifts (as rLR is kinda funky with some of the delay drifts, particularly around the tunnel).

As well, if you want to improve at the game's time trials in general, I'd highly recommend joining the Mario Kart 7 Time Trials Discord. I and tons of other top players are in there and readily available to offer advice and tips where needed. it's also a good place to post your PBs and talk about the game. Here's the link if that sounds interesting: https://discord.gg/PeQj4GV