Increase Speed on Every Frame

You are increasing in class constructor, which only executes once.

Override event Tick instead. And multiply by frame time to make it framerate independent. Eg :

void APlayerChar::Tick(float DeltaTime)
{
    Super::Tick(DeltaTime);

    movSpeed += incrSpeed * DeltaTime;
}

This will increase movSpeed smoothly by 0.25 per second, regardless of frame rate.