I spent my early career (circa late 2000s) deploying to various AS/400s, which are midrange machines. Up-front caveat: I used them for the most boring business-y work: querying databases, moving save files around, modifying some job queues; no computationally intensive tasks. I also worked from a modern laptop and only used the greenscreen for specific tasks. However, I spent a lot of time with people for whom an AS/400 terminal was their primary workspace.
1. My impression of user interaction was snappy. Power users absolutely flew from screen to screen. Navigation and most tasks are almost always instantaneous (~0.1s); blips into the ~1s range were rare enough to be notable. For the most part, job priority and time slicing just worked. The ceiling for fluid human-computer interaction in the greenscreen always seemed higher than in modern operating systems. I saw three reasons: the rich API surface of keyboards, the ubiquity & consistency of keybindings, and being blessed with the constraint of text screens. I feel we genuinely lost something here moving to mouse- and icon-oriented systems, and with speciation of UI commands across applications. I've seen several greenscreen->GUI modernization projects where the actual user experience regressed severely, despite prettier screenshots, in terms of responsiveness and access.
2. My memory is vague when it comes to how long program compilation took. A few seconds? (The odd architecture of the tech I worked on made this a non-issue: we would develop in an IDE on our laptop, compile part of it locally, and then send the program object to the AS/400 to compile another part and bundle it for execution. Local compilation (seconds to minutes) and transfer time (some seconds) by far dominated this workflow — live programming it was not.)
3. Only tangentially related: I always like to mention that these machines are shockingly reliable. I worked with many dozens of them and my company was involved with hundreds more and I never even heard of a crash or a machine getting into a bad OS state. Absolutely rock solid.