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:
Clean up the SFDC database to reduce the amount of contacts and leads you have in the system. Start by deleting junk and test records. Merge duplicates if possible.
Implement a custom sync rule to limit the number of contacts and leads which will sync.
I recommend preventing records without an email address from initially syncing with Marketo, since Marketo uses email address as the unique key for de-duplication. Setup workflow rules in SFDC to enable the record to sync once a sales rep enters an email address for the record.
If you don’t use tasks in Marketo, you can with support to disable tasks from syncing.
Here are some additional resources for learning about the Marketo integration with SFDC.