Hi Jakub,
Thanks for getting in touch. Unfortunately, what you’re experiencing isn’t really something that can be improved. The import process is quite complex, so the inaccessibility and slow speed are kind of par for the course when importing that many events. I apologize for the inconvenience, but there isn’t much I can do to help you out.
You might try increasing the PHP memory allocated to WordPress? You can do that by using one of the methods listed here: http://www.dailyblogging.org/wordpress/increase-wordpress-memory-limit/ That may make a difference, although especially when you add even more events it will still likely take a bit.
Best,
Leah