The Scheduler is optimized and it only renders the cells for the current viewport. However, this configuration extends the viewport to the whole grid and it displays all 100*15*24 = 60,000 cells at once. No browser is able to render it fast.
You need to limit the viewport vertically (this will also let you fix the headers during scrolling):
https://doc.daypilot.org/scheduler/height/
A timeline of 15*24 = 600 cells (horizontally) - when displayed on a single screen - will not only make the rendering slow but it also makes the content unreadable - I recommend using scale: "Day" instead.