Just had a question, if we displayed a number of different recommendations on a retail homepage or product page. i.e Recently viewed; category affinity; customers also viewed; popular products from favourite brand.
What you are describing sounds exactly like the intended use case for Recommendations:
Recently Viewed Customers also viewed. Popular products from favorite brand etc...
Typically I've seen about two of these on most pages but I don't think it's a decision made due to slowness rather I think it comes from not wanting to overwhelm the customer with too many options. Recs should be able to handle this.
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I would also add that performance largely depends on how you implement. If you run all these activities through VEC, then you have one call on the page that will take 60-100ms per qualified activity. If you broke them up into individual calls, then you have one call per activity at around the same performance. It just depends on how you want the network traffic to look.