Slow refresh when using Analyze in Excel for some region

Hi, we created a data model in Power BI and used as a cube accessed from Excel, by using Analyze in Excel.  But some users in North America are experiencing very slow speed and even seeing error message when refresh the data source in Excel, while users in other regions like Europe and Asia don't see the same thing.

• the file they created were based on the same power bi data model, but they copy paste it and built several tabs for customized views.  when hitting the refresh button, messages of 'waiting for the query to be excecuted' and 'Refreshing the OLAP cube' appeared couple of times, does that mean pivots are refreshed one by one rather than data source being refreshed one batch and pivots being updated?  counts of pivots will impact total refresh time?
• for the same file, people from North America usually take more than 15 mins to refresh, and see error messages frequently, but people from Asia/Europe only take 3~5 mins to refresh.  How the data is stored in Power BI services?  does it have distributed server in different regions?  Any way to troubleshoot which server users are hitting exactly and test the speed?

Thanks in advance!