You need to provide more information. - Which version of Xmind are you using? The newest, Xmind 8 update 5? - How much memory you have on your Mac? - Where you encounter the lag? What are you doing? How big is your map?
Search this forum for memory issues that have plagued a couple of recent versions. One reason for lag might be big editing history, search also this forum how to purge it.
This is a common error: I eventually had to purchase SimpleMind so that I could get a functional mind mapping app. However, there is a beta of a new version for download here: https://www.xmind.net/zen/
It works well, and offers promise for the future, but I hope it will be a free upgrade for this of us who paid for Mind 8 but can't use it.