IanaToWindows performance improvement

private static string IanaToWindows(string ianaZoneId)
{
    if (string.IsNullOrEmpty(ianaZoneId))
        return null;

    var utcZones = new[] { "Etc/UTC", "Etc/UCT", "Etc/GMT" };
    if (utcZones.Contains(ianaZoneId, StringComparer.Ordinal))
        return "UTC";

    var tzdbSource = NodaTime.TimeZones.TzdbDateTimeZoneSource.Default;

    // resolve any link, since the CLDR doesn't necessarily use canonical IDs
    List<string> possibleZones = tzdbSource.CanonicalIdMap
        .Where(x => x.Value.Equals(ianaZoneId, StringComparison.Ordinal))
        .Select(x => x.Key)
        .ToList();

    // resolve canonical zones as well
    string canonical;
    if (tzdbSource.CanonicalIdMap.TryGetValue(ianaZoneId, out canonical))
    {
        possibleZones.Add(canonical);
        possibleZones.Add(ianaZoneId);
    }

    // map the windows zone
    var mappings = tzdbSource.WindowsMapping.MapZones;
    var item = mappings.FirstOrDefault(x => x.TzdbIds.Any(possibleZones.Contains));
    if (item == null)
        return null;

    return item.WindowsId;
}