Having a lot of SFDC records can possibly impact the Marketo sync speed. I define a large SFDC instance as someone with more than 1M records based on the number of Account, Contacts and Leads they have. The first step in optimizing sync performance is to run counts in SFDC on the number of Accounts, Contacts, Leads, and Tasks.
Here are possible options to improve performance:
Here are some additional resources for learning about the Marketo integration with SFDC.