Scaling Performance Considerations

I'm in the process of designing our strategy for using Zoho CRM in our operation. One question I have...we're going to have more prospects/lead than contacts (obviously people who convert from a website are a fraction of total visitors). We're trying to decide if we should front-end our Zoho CRM implementation with a separate database for these "prospects" and when they convert, house the customer record in Zoho. The challenge is integrating a separate database to our other downstream systems (as opposed to tie-ing everything into Zoho). My concern with keeping EVERYTHING in Zoho, will be the sheer amount of data we could end up propagating into the CRM. Are there any performance considerations with scaling with Zoho? If we end up with millions of records, should we expect any performance degradation with using the platform? ...and if there are scaling/performance limits, at what point (number of records) do they tend to show themselves?