I don't know how many concerts you've been to, but music is perceived very differently based on the external circumstances. What seems like a perfectly fine tempo for a song in a calm living room environment, will be perceived as plodding in a more adrenaline-driven environment such as a live concert.
Dismissing the state of the listener, the performers, and the environment they are in as "silly", is frankly rather ignorant about humans work.
I've been to plenty of concerts.
Regardless of how things might be perceived, I just think that saying, "They should play their songs faster live than in the studio" is kind of a silly thing to expect. Some bands choose to do it, some don't, some only do it for certain songs, others might do it for every song, but it's certainly not something that should be considered mandatory.
Sometimes it works, sometimes it doesn't. I honestly haven't even noticed it for The Glass Prison on the Gigantour DVD, but apparently, someone said they're playing it faster, I don't know. But sometimes it is noticeable, and when it is, it feels like the song is being rushed through, even if it's to drive the energy up.
But regardless, for the past two Dream Theater tours, never once have I felt like anything was plodding or dragging. Even songs that I wasn't particularly excited to hear never dragged for me.