When downgrading IDEA versions using JetBrains Toolbox the VM config is lost/reset.
The IDE has started with default settings, including a memory limit of 750 MB. It took me 15 minutes to figure out why my IDE is suddenly almost unusable and keeps freezing. Even just trying to reach the Help menu to increase the setting takes minutes and my entire MacBook hangs 😮
I suggest to set the default memory limit much higher.