Update cross-workspace-query.md

Author: guywi-ms

articles/azure-monitor/logs/cross-workspace-query.md

@@ -30,6 +30,7 @@ There are two methods to query data that's stored in multiple workspaces and app
 ## Cross-resource query limits
 
 * The number of Application Insights components and Log Analytics workspaces that you can include in a single query is limited to 100.
+* Querying across a large number of resources can substantially slow down the query.
 * Cross-resource queries in log alerts are only supported in the current [scheduledQueryRules API](/rest/api/monitor/scheduledqueryrule-2018-04-16/scheduled-query-rules). If you're using the legacy Log Analytics Alerts API, you'll need to [switch to the current API](../alerts/alerts-log-api-switch.md).
 * References to a cross resource, such as another workspace, should be explicit and can't be parameterized. See [Gather identifiers for Log Analytics workspaces](?tabs=workspace-identifier#gather-identifiers-for-log-analytics-workspaces-and-application-insights-resources) for examples.