Thanks for contacting us.
300 000 images could take a while, the plugin needs to index them to perform operation on it later. Does the progression stops or i keeps going?
You also have the option to index them by sequence: select only half or 30% of your media from the plugin configuration then run the optimisation. Once i's done you can continue with the rest.
If you feel the delay is really too long you can drop us a ticket, the developer will take a look at it. Keep in mind that 300K images is a huge amount of data, even for a server.