I am experiencing the same issue. I am a new user of Xmind and I am trying to use it on two iMacs both running High Sierra. One is a year old and the other is brand new and both have the same spec - 3GHz processor and 8GB memory and 2GB video memory. The software is completely unusable and I am spending more time correcting typing mistakes caused by the lag than I am doing what I wanted to do in the first place. I am very much regretting having spent money on the Pro version.