Why does CiraSync take so long to sync my contacts and calendars?

There are multiple factors that can affect CiraSync performance. These include the following:

• Number of mailbox in a tenant
• Amount of updates that need to be made
• Type of data that needs to be written in
• Current performance of Exchange Web Services (EWS)

Below are steps you can take to speed up CiraSync sync tasks:

1. Set up your CiraSync tenant to use multiple service accounts.
2. Enable the CiraSync streamer in the Schedule pane of the CiraSync dashboard if the sync source is a shared mailbox or a Public Folder. CiraSync will then periodically read the source and then sync any changes found to user mailboxes. 
   NOTE: Unfortunately, CiraSync has no control over the performance of EWS, and no workaround exists to improve its performance.

If you want more scheduled updates to occur, check out this article on The CiraSync Streamer and How to See Contact Sync Result Reports.