CSS or and JS aggregations all makes site slower! It is well known among Drupalists that generally, Aggregation of CSS\JS files should hasten webpage loading times in Drupal... So why the opposite is true for my site this time?


PSI test score for my Drupal 7.37 site after No Aggregation whatsoever: 72/100
PSI test score with CSS aggregation only: 59/100
PSI test score with JS Aggregation only: 70/100
PSI test score with Both file types
aggregation: 67/100


Bottom line: the natural aggregation options do the opposite of their inherit aim --- They are slowing my site instead of hasting it! Any reasons you could suggest for such wired behavior? Thx.
You're confusing PSI and webpage loading times; they are not the same thing. 

PSI is series of benchmarks for optimizing performance. It's a checklist Google provides to give you suggestion for improving performance. It doesn't gauge what your site's actual speed is. Use your browser's network activity inspector if you'd like to analyze site load times against CSS/JS aggregation.