使用SQL Server数据库调用的c#多线程

I have a large DataTable that contains Users details. I need to complete user's details into this table from several tables in DB. I run though each row in the table and make several calls to different tables in the database, using ADO.NET objects and methods, process and reorganize the results and them to the main table. It's works fine, but to slow... My idea was to split the large table into few small tables and run the CompleteAddressDetails method in a few threads simultaneously and in the end to merge small tables into one result table. I have implemented this idea using Task object of TPL. There is a code below. It works fine, but without any improvement of execution time. Several questions: 1. Why there no any improvement of execution time? 2. What I have to do in order to improve it?I have a large DataTable that contains Users de