Hi ranjeet
If you are using multiple iterations in your design but you wish to check the output only for a specific iteration,buffer selector can be used to select the required iteration.
But I don’t think the system should take longer to simulate just because you are using a selector in your design.Anyway if that is an issue you can run your simulation without the selector because anyway its significance is to filter out the required iteration to make the result evaluation easier.
And for BER and Q.Factor US results increase the signal index which you can find on the right top extreme of the analyser window.
Hope all this helps
Regards