Would there be a way to remain optimized when using MAX()?

I have a select like this:

SELECT TOP 10 Client.NoCountry,MAX(Client.Numero)
 FROM Client
 GROUP BY Client.NoCountry ORDER BY 2 DESC

Without the MAX(), the select runs instantly. With the MAX(), despite the fact that I created the execution plan recommended index, there is still a delay. I am looking for alternative approaches which would achieve the same.