We've been conservative in opening up SFDC fields to sync to Marketo and in creating new Marketo-native fields. My understanding from Marketo is that the more fields we create, the heavier the database gets... resulting in various operations getting slower.
So I'm wondering how conservative I really need to be, since I don't really want to hinder marketing operations just for the sake of database efficiency. But I also don't want to hinder marketing operations by having an inefficient database. 🙂
So I'm curious how many fields others have (and I guess it's important to note how many records you have), and if you've noticed performance hits as you've added them in.
You are right to be a little conservative, I have worked with Marketo users before who have had 100s of fields per object and this was increasing the average sync time and increasing the likelihood of a backlog of updates. Certain fields such as formula fields and roll up summary fields may also take longer to sync.