If I try to increase the max speed of the light speed rail above 1.0 in the config file the minecart is never loosing its speed when it reached its max speed.

Minecraft 1.20.1
Forge 47.3.10
More Minecarts and Rails 1.8.3

I face similar issue.
I use same version's Minecraft, Forge and More Minecarts and Rails.
I putted 1 enabled Turbo Rail and 9 Lightspeed rail, and I putted 10 enabled Turbo rail at start of tracks. And I updated cart to Aerodynamic Minecart.
And I set lightspeed_rails_max_speed to 10.0.
But top cart speed is not reached to 10m/tick. it only up to approximately 1.5m/tick.

This is just a thought, we can't up the cart speed much higher than 1m/tick due to limit by Minecraft?