Reasons for slow Dashboard Refresh speeds

All source reports must finish before a dashboard can be refreshed. 
Each individual dashboard component's source report must be run, calculated, and transformed into the dashboard widgets whenever you refresh a dashboard. The more complex your source reports are, the longer the overall dashboard will take to refresh. For best practices and troubleshooting, see our Improve Report Performance: Best Practices help article.

Dashboard refreshes are a queued process.
All dashboard refreshes for your organization are funneled into a queue, based on the server where your organization resides. This means that multiple customers can be requesting refreshes on the same server at the same time. Each dashboard refresh (per server) is handled on a first-in, first-out basis, meaning that if there are large refresh requests in the queue before your request, it can cause occasional delays in the refresh process.