I have created a Drupal Views block, which aggregates some CiviCRM values. This block I have on most pages.
How (if any) is this affecting the performance of the CiviCRM site?
(How does Drupal Views aggregate its result? SQL query?)
Views has an option to display the exact query it generates, so you can get a rough idea by running that query alone with a tool that allows you to time it. Ideally a single query should execute in a matter of milliseconds, not seconds.
If you have general concerns about sql performance, enable slow query logging to find out which queries on your site are taking the longest.
Views also offers 'caching' so you could set that if you do have proof that it is the View display that is slowing down your page load.