How do aggregate projections affect the load performance?

They affect the load performance like any additional projection.
If you come from another database than, think of adding indexes, and the effect of that - or , for DBMSs that support that, incrementally updating an aggregate Materialized View.
So if you update your base table once a day and run one aggregate report once a day, you might not want to do it. But if you have hundreds of reports daily needing the pre-aggregated data, you will gladly invest the additional workload at load time.