Hi,
The problem with querying large lists is that it will run multiple queries in batches of 5000 items – in your case 6 queries. If you bring up the developer console and look at the network tab when you load you should see 6 individual queries.
Do you also see a red line with status 500? – if so, this confirms that SharePoint enforces the list throttling limit of 5000 items for external list as well.
I’m not sure there is a good solution to this issue, but maybe some of the forum users can share some additional insight?
Alexander