private void CalculateExtendedDiskUsage(IEnumerable<Configuration> allConfigurations)
{
    var sw = new Stopwatch();
    sw.Start();
    // Lets take only confs that have been updated within last 7 days
    var items = allConfigurations.AsParallel().Where(x =>
        x.artifact_cleanup_type != null && x.build_cleanup_type != null &&
        x.updated_date > DateTime.UtcNow.AddDays(-7)
        ).ToList();

    using (var ctx = new LocalEntities())
    {
        Debug.WriteLine("Context: " + sw.Elapsed);
        var allBuilds = ctx.Builds;
        var ruleResult = new List<Notification>();
        foreach (var configuration in items)
        {
            // all builds for current configuration
            var configurationBuilds = allBuilds.Where(x => x.configuration_id == configuration.configuration_id)
                .OrderByDescending(z => z.build_date);
            Debug.WriteLine("Filter conf builds: " + sw.Elapsed);

            // Since I don't know which builds/artifacts have been cleaned up, calculate it manually
            if (configuration.build_cleanup_count != null)
            {
                var buildCleanupCount = "30"; // default
                if (configuration.build_cleanup_type.Equals("ReserveBuildsByDays"))
                {
                    var buildLastCleanupDate = DateTime.UtcNow.AddDays(-int.Parse(buildCleanupCount));
                    configurationBuilds = configurationBuilds.Where(x => x.build_date > buildLastCleanupDate)
                            .OrderByDescending(z => z.build_date);
                }
                if (configuration.build_cleanup_type.Equals("ReserveBuildsByCount"))
                {
                    var buildLastCleanupCount = int.Parse(buildCleanupCount);
                    configurationBuilds =
                        configurationBuilds.Take(buildLastCleanupCount).OrderByDescending(z => z.build_date);
                }
            }

            if (configuration.artifact_cleanup_count != null)
            {
                // skipped, similar to previous block
            }

            Debug.WriteLine("Done cleanup: " + sw.Elapsed);
            const int maxDiscAllocationPerConfiguration = 1000000000; // 1GB
            // Sum all disc usage per configuration
            var confDiscSizePerConfiguration = configurationBuilds
                .GroupBy(c => new {c.configuration_id})
                .Where(c => (c.Sum(z => z.artifact_dir_size) > maxDiscAllocationPerConfiguration))
                .Select(groupedBuilds =>
                    new
                    {
                        configurationId = groupedBuilds.FirstOrDefault().configuration_id,
                        configurationPath = groupedBuilds.FirstOrDefault().configuration_path,
                        Total = groupedBuilds.Sum(c => c.artifact_dir_size),
                        Average = groupedBuilds.Average(c => c.artifact_dir_size)
                    }).ToList();
            Debug.WriteLine("Done db query: " + sw.Elapsed);

            ruleResult.AddRange(confDiscSizePerConfiguration.Select(iter => new Notification
            {
                ConfigurationId = iter.configurationId,
                CreatedDate = DateTime.UtcNow,
                RuleType = (int) RulesEnum.TooMuchDisc,
                ConfigrationPath = iter.configurationPath
            }));
            Debug.WriteLine("Finished loop: " + sw.Elapsed);
        }
        // find owners and insert...
    }
}

Context: 00:00:00.0609067
// first round
Filter conf builds: 00:00:00.0636291
Done cleanup: 00:00:00.0644505
Done db query: 00:00:00.3050122
Finished loop: 00:00:00.3062711
// avg round
Filter conf builds: 00:00:00.0001707
Done cleanup: 00:00:00.0006343
Done db query: 00:00:00.0760567
Finished loop: 00:00:00.0773370

foreach (var configuration in items)
        {

            var confDiscSizePerConfiguration = await GetData(configuration, allBuilds);

            ruleResult.AddRange(confDiscSizePerConfiguration.Select(iter => new Notification
            {
               ... skiped
        }

private async Task<List<Blabla>> GetData(Configuration configuration, IQueryable<Build> allBuilds)
        {
                var configurationBuilds = allBuilds.Where(x => x.configuration_id == configuration.configuration_id)
                    .OrderByDescending(z => z.build_date);
                //..skipped
                var confDiscSizePerConfiguration = configurationBuilds
                    .GroupBy(c => new {c.configuration_id})
                    .Where(c => (c.Sum(z => z.artifact_dir_size) > maxDiscAllocationPerConfiguration))
                    .Select(groupedBuilds =>
                        new Tmp
                        {
                            ConfigurationId = groupedBuilds.FirstOrDefault().configuration_id,
                            ConfigurationPath = groupedBuilds.FirstOrDefault().configuration_path,
                            Total = groupedBuilds.Sum(c => c.artifact_dir_size),
                            Average = groupedBuilds.Average(c => c.artifact_dir_size)
                        }).ToListAsync();
            return await confDiscSizePerConfiguration;
    }