I would like to add 1500 url redirections using .htaccess on a apache server. Is it a bad idea ? will it decrease the performances significantly?
It would probably work but it will be difficult to maintain. Performance will start to take a hit as you increase the number of directives, so if there are patterns in your redirects, you may look to bundle them into a single directive (if you can).
It would be architecturally superior to knock together a quick script that looked up the redirection from a more-maintainable and indexed file format (or preferably a database).