I have Windows Service that accesses Outlook items through Exchange Web Service (EWS). It often experiences concurrency issues.
Based on this doc, a maximum of 27 connections per user can be made concurrently for Exchange Online. I couldn't find documentation for the detailed throttling policies for Microsoft Graph.
Is there anyone know if I can make more concurrent connections in Graph API than EWS? Also, does Graph API show better performance in general than EWS such as response time?
This is documented in Microsoft Graph throttling guidance. There is also an in-depth look at Throttling patterns available.
Each endpoint throttles a little differently. For Exchange/Outlook endpoints it is 10,000 requests per app id, per user, within a 10 minute window.
The term "connections" doesn't really apply to an API like Microsoft Graph. As a REST-based API, it is fundamentally stateless. Each request is a self-contained transaction.