1500 tabs easily touches enough RAM that average computers will keep swapping if that's actually your working set. Which is why any good browser should be completely freezing tabs that haven't been active in a while so their memory can remain swapped out, or even completely unloading them.

Though, 1500 tabs in one window is beyond the point at which it impacts Safari's responsiveness, but that's completely unrelated to the contents of the tabs and is instead the fault of taking way too long to update every NSView in the tab bar.