Performance data stays on the slave all the time. Only HW/SW inventory data is transferred to the master.
For such a high amount of devices (SNMP devices need more CPU than agent devices) i would split the devices on more than one instance.
RAW edition has a “soft” limit per instance i would say around 1k hosts (this is no real hard limit).
What you can do is have a very heavy iron as your monitoring server and run several instances on this one machine. If you have enough resources it is also possible to handle 8k hosts on one machine.