I don't understand the problem. 2000Hz = 0.5ms/frame; 20% of this = 0.1ms. That sounds like a great result. Your target frame rate is presumably 60-100Hz, assuming it's a PC game, meaning your frame budget is 10-16ms. If your UI takes 0.1ms, you've got >99% of your budget left.
(Also: 3-4K vertices for UI was about what you could expect to budget for a PS2 or Xbox game! - max throughput for PS2 was something like 250,000 vertices/frame at 60Hz, and this is <2% of that. I struggle to believe this is any kind of an issue for anything modern.)