I presume you're talking about the admin interface on http://localhost:8080/_ah/admin/? If you want to insert many entries, you should be using code, or the bulkloader. There's no limit on the number of entries you can insert using those methods.
This won't be particularly illustrative, though: The performance of the dev_appserver datastore is not representative of the performance of the production datastore. dev_appserver currently stores data in memory, so once you exceed what can be held in your physical RAM, it will slow down dramatically. Obviously, nothing similar applies to the production datastore!