Are Coinpayments APIs suitable for generating many payment addresses?

[Coinifyx]

I was testing Coinpayments APIs today and I noticed that after few times of calling get_callback_address the generation became slower, up to the point it would take even 10-15 seconds to complete...

Has anyone experienced the same during development? Is this a limitation of the site/I need to upgrade in some way?(It doesn't seem like that)

Sometime it works fast and it goes unnoticed sometime it takes a while to generate one I am not sure if this is suitable for a project with many addresses/buyers at same time, if someone had previous experience with this please let me know, maybe is related to specific cryptos?