I wonder if there's a way it could be coded to keep the window responsive if the user wants to move.
I'm sure there is, but I don't know of a good way. Maybe it could break up the SQL query into smaller requests and respond to the UI in between. Bottom line, JS is single-threaded, and when JS code is executing, your browser window's not doing anything else until the JS code yields control back to the browser UI. When it's taking in a huge chunk of data from the DB, processing it, and adding thousands of things to the Leaflet map, it's going to look like it's hanging. Given enough memory for your browser and patience of the user, you can pretty much load anything you want. It's just going to take a long time.
TM's been in somewhat desperate need of a true web developer since the start. An analysis and possible redesign by DB expert wouldn't hurt either. I've pieced a lot of things together over these 8 or so years but I'm the first to admit that I'm no web development or database expert. If someone were willing to do for the DB and the web front end what yakra has done for the site update process, I am sure we would have a pretty incredible site.