In fact, importing a database into another database can take pretty long! This depends mainly on 1) the amount of memory allocated to your openLCA installation 2) the speed of the hard drive where databases are saved (SSD/HDD?) 3) the size of the databases.
When importing one database into another, openLCA checks the imported data sets for interdependencies with the existing database. If both databases have e.g. more than 10,000 processes this can take really really long!
Can you maybe add to your question which databases you are trying to combine and in which file format?
Update: I am not really sure if it makes sense to combine ELCD and Exiobase but of course I would always recommend to import the smaller database into the larger database.
Restore Exiobase (there is no reason for importing Exiobase into an empty database) which should be quite quick and then import methods and the ELCD database. That should be the quickest option which, depending on your setting, can still take quite some time. Do not import the Exiobase into an existing database.