Hi Jacek,
Sorry to hear about the troubles. Are you importing a lot of events from Facebook? If there’s a lot, this could result in a slow down. Is it still slow after the import has finished? How many events total do you have on your WordPress website? It could be a conflict with another plugin, please try deactivating all other plugins to see if this helps.
You might also want to look at your server/hosting setup. What web host are you with? Have you tried 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/
Finally, another thing to look at is your theme. You should try reverting to the Twenty Eleven/Twelve theme to see if that helps.
Let me know if any of the above helps.
– Jonah