Hi Zdenek Just to be clear, BIG-IP Next will not have the same control plane challenges with API calls that exist with classic BIG-IP.
The issue at scale with both AS3 (which is an iControl LX package) and iControl REST is the shared restjavad process, so a lot of calls with heavy payloads will be impactful either way and need to be managed accordingly. If you aren't having issues today, I don't anticipate that will change with your shift to AS3, but testing in a lab on large configs will be helpful to determine that in your environment.