Remove the elytra speed limit. Fly faster, and keep getting faster.
Elytra Unleashed removes both things that cap elytra speed: glide drag and the firework speed limit. Fireworks and dives keep adding speed with no ceiling. Fly too fast for too long and you catch fire.
In vanilla Minecraft, elytra flight constantly slows you down. Every tick your velocity is scaled down a little, so no matter how many fireworks you use or how steeply you dive, you hit a terminal speed and level off.
Fireworks have a hidden limit too: a rocket pulls you toward a fixed speed, so past it, they actually slow you down.
Elytra Unleashed removes both caps. Glide drag is gone, and rockets keep pushing you forward no matter how fast you already are:
Speed only stops growing when you stop gliding, land, or crash.
Glide faster than 60 m/s for 3 seconds and you overheat. You burst into flames and trail fire and smoke for as long as you hold that speed. Slow down, hit water or rain, or drink Fire Resistance to survive.