由于左连接，linq缓慢声明？或分组

Question

嗨我在linq查询下面但是它很慢......我在做左连接时它很慢但是不能真正判断是否是这种情况。

 var lastPayment = (from p in _context.CustomerPayment
                           group p by p.CustomerId into grps
                           select new
                           { PaymentID = grps.Max(x => x.PaymentId) });


        List<CustomerPaymentViewModel> data = await (from cp in _context.CustomerPayment
                                                     join lp in lastPayment
                                                     on cp.PaymentId equals lp.PaymentID into _l
                                                     from lp in _l.DefaultIfEmpty()
                                                     orderby cp.PaymentId descending
                                                     select new CustomerPaymentViewModel()
                                                     {
                                                         CustomerName = cp.Customer.CustomerName,
                                                         PaymentAmount = ((decimal)cp.Payment.PaymentAmount).ToString("n0"),
                                                         PaymentDate = cp.Payment.PaymentDate.ToString("yyyy-MM-dd"),
                                                         PaymentID = cp.Payment.PaymentId,
                                                         OutstandingAmount = cp.OutstandingAmount == null ? "" : ((decimal)cp.OutstandingAmount).ToString("n0"),
                                                         PaymentMethod = cp.Payment.PaymentMethod == null ? "" : cp.Payment.PaymentMethod,
                                                         Editable = lp != null ? "1" : "0"
                    }).ToListAsync();