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.