Form Rendering Enhancements
Forms have been enhanced so that they load faster. However, the enhancements can cause problems with unsupported customizations. Form enhancements can be turned off while unsupported customizations are addressed.
The setting is on System Settings | General (tab) and is named Use legacy form rendering
More details at this MSDN blog, http://blogs.msdn.com/b/crm/archive/2015/04/29/microsoft-dynamics-crm-online-2015-update-1-new-form-rendering-engine.aspx
It is likely that this setting will be removed in future releases, so it is important to address all unsupported customizations.