I did research for this game in 2020 and I'm currently looking at it again.

In 1A, the first boost as well as the boost after entering the ground, I was not able to gain any speed by pausing (regardless what timing and how long I wait until I unpause each time), but it's easily possible to keep speed for longer by spamming the B button so the game does a "no nitro sound".

53.92 is the best possible time in TAS at the time of writing, and it should be possible for RTA to reach times better than 54.06 (the current RTA WR) or 54.04 (the best time RTA could achieve without pausing and without using "no nitro sound") easily.