为什么在填充表后索引表时SQL Server的工作速度更快？

I have a sproc that puts 750K records into a temp table through a query as one of its first actions. If I create indexes on the temp table before filling it, the item takes about twice as long to run compared to when I index after filling the table. (The index is an integer in a single column, the table being indexed is just two columns each a single integer.)I have a sproc that puts 750K records into a te