I investigated the performance when starting JabRef and found the following things that can be improved:

• At the moment, opening a file blocks the JavaFX thread which makes the impression that JabRef is frozen. Instead an empty database should be opened immediately (with the correct name), which shows a progress indicator in the maintable until the database is parsed. In particular, files should be parsed in a background thread and not on the JavaFX thread.

  This applies also to the files that are opened at the beginning: This is fixed with Performance improvements for GUI start up #5423

• When opening a file,
  

  jabref/src/main/java/org/jabref/gui/JabRefFrame.java

  Line 1010 in 311afa3

  BackupManager.start(context, Globals.entryTypesManager, prefs);

  
  invokes the backup manager, which creates a backup file. This shouldn't be necessary as there were no changes to the file yet.

• The group side panel should be converted from fxml to plain Java code (parsing fxml takes about 500ms)

• When opening a database, the autocompletion is initalized on the main thread.
  

  jabref/src/main/java/org/jabref/gui/BasePanel.java

  Lines 743 to 747 in 311afa3

  	// Set up name autocompleter for search:
  	instantiateSearchAutoCompleter();
  	this.getDatabase().registerListener(new SearchAutoCompleteListener());
  	setupAutoCompletion();

  
  This should be done in a background thread. This is fixed with Performance improvements for GUI start up #5423.