There is a few possibilities here.
You could have a delay on SFDC API object calls due to:
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Lots of data being updated
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other systems connected to SFDC updating several times a day
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MSI emails through outlook are not logged correctly by rep (rep forgetting to select 'log with Marketo' for replied email)
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Sales more comfortable with SFDC outlook plug in
We have had issues with data not syncing correclty with SFDC and found that we had Data.com upddating 3 times a day so the Marketo/SFDC sync was back logged for weeks trying to catch up.
Also, If memory serves, I'm sure only one type of activity gets sent once per day for that type of activity.
Are you merging in SFDC and the update taking a while in Marketo or vice versa?