I can say from experience that I’m definitely the type of person who gets upset with myself when I mess up a part I’ve played hundreds of times, and I’ve also had discussions with bandmates about being mindful of tempos when it comes to songs they start and also have developed facial cues for the drummers I play with to let them know to bring the tempo down or up when a song is rushing or dragging. It’s more than just “oh I messed up this part because the tempo was too fast”. People forget that playing music is a physical thing, and if your body is trained to perform something one way and then suddenly you have to adjust, it could be uncomfortable. I’ll give two examples in my band.
The first example is Come on Eileen. My band does the ska version, which is a bit faster than the original. The way the bassline is fingered, it’s not that difficult but the continuous eighth notes and arpeggiated pattern can be tiring. My keyboard player often kicks into the song faster than normal, and that split second of time between notes that is lost makes a huge difference. If I don’t signal to the drummer to bring the tempo down, my hands will be dead by the end of the song.
The second example is Say So by Doja Cat. We play this song at a faster tempo so it grooves more and transitions more seamlessly into the next song since my band does blocks of songs with no pauses between them. Our old drummer would always play the song at album tempo because, in his words, “this song needs to lay back and groove”, but he was ignoring the fact that the longer vocal phrases meant our singers need more breath support to sing it, and after having played for over an hour, they were definitely tired. The faster tempo meant less time between breaths, which helped our singers immensely with staying in key and keeping a good vocal tone.
My point here is, while someone in the audience might not notice or care about slight mistakes that come with a tempo that’s slightly too fast or too slow, the performer’s body cares a lot, and especially with a touring band like Dream Theater, those tempos being off night after night, forcing them to play or sing parts at a different speed than their body is used to, it could lead to more muscle and voice fatigue than they would have experienced otherwise. For us fans, it’s one night. For them, it’s weeks and months at a time, so those perceived little things become a lot bigger deals as the tour goes on.