Below 15mph RR (Road-Rolling Resistance) is the major resistance factor.

Somewhere above this speed AR (Air-Wind Resistance) becomes the major resistance factor.

While RR increases at a linear rate -1, 2, 3, 4, 5, 6, 7, 8, ...

AR increases at a geometric rate - 1, 2, 4, 8, 16, 32, 64 etc.

This graph shows the resistance transition occurring near 18km/h (~11mph)

This appears to be a road or race bike.

Air resistance is such a factor that at near 30mph ...

An electric bike with lowered seat and crouched rider is faster than a bike with a "properly" positioned rider applying substantial assist.

A Mountain bike requires 255w of pedal effort to maintain 18mph.

I chose 18mph because most cyclists are familiar with the effort required to maintain this speed.

This graph demonstrates the additional speed available from adding the effort required to power a bicycle at 18mph.

From 10mph, assist will double speed to 20mph (10mph).

From 20mph, the same amount of pedal assist will increase speed 5mph.

From 30mph, the same amount of pedal assist will increase speed 2.5mph.

From 40mph, the same amount of pedal assist will increase speed <2mph.

**However at 50mph, the same amount of pedal assist will increase speed ... barely ... 1mph! **
On the other hand ...

This degree of pedal assist will more than triple range (300%) at 20mph and almost double range (200%), cruising at 25mph.

At 30mph assist increases range, possibly 33% (133% total).

Pedal assist contribution percentage declines quickly with speed.

Near 30mph, a more aerodynamic position outperforms even substantial pedal assist from a "proper" pedal seating position.