Well I have had reason recently to suggest that even bent valves work better at higher revs , a customer brought a 1.6 focus black top to me with a low speed misfire , it had suffered a broken cam belt at idle it even struggled to run , but as soon as you lifted the speed up to above 2000 rpm it ran on all 4 .

The same thing was happening on the ka , at higher speed the combustion gases cannot escape fast enough through the valves but at low speeds it caused low compression .

Just my 2 pence worth lol .