Update June 28, 2018
Note that this issue has been addressed in the Reason/Intro/Lite 10.1.1 and Reason 9.5.3 updates. Please have a look at this article for more information.
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It has come to our attention that Reason does not work properly under the latest macOS High Sierra 10.13.4 update. Currently, we do not recommend you to update your macOS High Sierra to 10.13.4 if you rely on using Reason, before we have fixed this.
We’re investigating this issue right now and while we currently don’t have a solution, we hope to get to the bottom of this.
If you have already updated to macOS High Sierra 10.13.4 and are experiencing performance issues, try deactivating the "Use hyper-threading audio rendering" function in Preferences - the General tab. This might improve the performance in some cases.