Well, this is expected because, during the initial sync, Marketo is copying everything that it can see and has access to from Salesforce. The greater the number of items to sync during the initial sync, the more time required for it to complete. Keeping this in mind, one way you can greatly reduce the initial sync time is by choosing to sync the necessary items first (Lead, Contact, Account, and Opportunity objects), and then gradually include the rest of the items (e.g., custom objects records, campaign sync, etc.) once the initial sync is complete. However, IMHO I'd recommend giving all the access you deem right to the sync user at first and then waiting for the initial sync to complete. You should also consider reading this article: Marketo / Salesforce Sync Limits & Priority Info.
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