This message is displayed when an Index Block in the system renders a large amount of data which can not be safely rendered due to resource constraints. The size at which the application will stop rendering an Index Block can be configured by doing the following:
Navigate to Administration > Preferences > Content.
Under Index Blocks, locate the field Maximum Rendered Size of an Index Block (MB).
By default this setting is 5MB. You can increase this value if your instance of Cascade CMS has sufficient resources to handle the resource demand. A good rule of thumb is to have about 5MB for every 1GB of RAM to which you have allocated to Cascade CMS. For example, if you have 3GB of RAM allocated to the JVM running Cascade, you can change the Maximum Rendered Size of an Index Block (MB) setting to 15MB.
If you aren't sure how much RAM is being allocated to your Cascade CMS instance, check out this article for identifying your current minimum and maximum heap sizes.