There is no limit for the Bex queries to embedded into the dashboard, but having said that you need to make sure you are connecting too many. Ex: more than 15. Its all depends upon the performance, if you have connected 15 queries with the dashboard you need to see how many are getting refreshed initially and how many are getting refresh based on the prompts all that matters.
Check the components whatever you used are mobile supported. Also please do the below step to identify where its getting delayed.
Resolution
- Open Dashboard within Dashboard Designer.
- Go to File -> Document Properties
- Check on "Show Loading Status"
- Save Dashboard back to BOE.
Hope this Helps!!!