>>Viv,
>>
>>After I switched to List and Linq, the performance is 10 times slower than it was :( The procedure used to execute in ~800 ms. Now it executes in 8000 ms.
>
>Then you've got something wrong.... linq is blazing fast.

Yes, looks like there is some bug here in my code as I see that the same line is executing again and again. I am going to go to the root of the problem - yesterday at night I was also first getting this problem and was able to solve it eventually.