unique ID question

[Evgeniy Strokin]

If I make one of my field as a unique ID, id doesn't increase/decrease 
performance of searching by this field. Right?
For example if I have two fields, I know for sure both of them are unique, both 
the same type, and make one of them as a Solr Unique ID. The general 
performance should be the same if I want to retrieve a document by first field 
or by the second.
Am I correct? Any general ideas or comments on this topic would be helpful to 
better understand how unique ID works.
 
Thank you
Gene