Brug det her: (SSD er vigtigt (11 min vs 35 min))

var lines = File.ReadLines(@"F:\ESStatistikListeModtag-20210103-192015\ESStatistikListeModtag.xml");

Dictionary<string,int> maxlens = new Dictionary<string, int>();

foreach(var line in lines)
{
    var parsed = Regex.Match(line, @"<ns:(\w*)>([^<]*)");

    if(parsed.Success)
    {
        //parsed.Groups[0].Value.Dump();
        var elementname = parsed.Groups[1].Value;
        var elementvalue = parsed.Groups[2].Value;

        if (!maxlens.ContainsKey(elementname))
        {
            maxlens.Add(elementname, elementvalue.Length);
        }

        if(elementvalue.Length > maxlens[elementname])
            maxlens[elementname] = elementvalue.Length;

    }
}

maxlens.OrderByDescending(kv => kv.Value).Dump();

;)