I also had a similar problem since I updated my OS X system. Increasing memory allocation to RevMan (or Java in general) solved the problem (see Bart Jourquin's answer). You need to restart your computer to see the change.
I was experiencing the same issue on macOS 10.15.4. Catalina. Following the suggestion above by Bart Jourquin, I did the steps described below and it runs smoothly now.
If RevMan 5 is running slowly, e.g. for very big reviews, here are some things you can try:
Turn off 'Validation as You Type' and 'Spell checking as You Type'
Limit the 'Undo History Size' under Tools > Preferences > General. You can even set it to zero if you don't use undo
Check that 'Save Backup File Every' under Tools > Preferences > Files is not set too low (e.g. every minute)
On Mac OS X you cannot specify the memory setting for RevMan 5 in particular but you can specify the setting for Java applications in general on the Java Preferences dialog. On later versions of Mac OS X, the Java Control Panel can be accessed from System Preferences instead of from Applications/Utilities. Click the Java tab, click View and add -Xmx1G under Runtime Parameters.