In Geneious Prime, go into Tools→Preferences, and under the General tab increase the “Max memory available to Geneious”. You may need to restart for the change to take effect. On Window Vista, Windows 7, 8 and 10 you need to run Geneious Prime as an administrator to change this setting (close Geneious Prime, then right-click on the Geneious icon and choose “Run as administrator”).