T28P and multiple EXT39 modules crash/lag the phone

[hunters]

Hi,
we have a customer with 120 Extensions to monitor as BLF.
We made several tests with T28P and 3 fully configured EXT39 modules attached.
We got a big problem starting when you attach the second module.
The phone start to not answer to many NOTIFY sip messages that are sent from the PBX (asterisk 10.x) and start to be lagged when you press keys on IT. Some time il reboot itself (may be some kind of watchdog inside the phone OS).
With the Firmware version 2.70.0.150 the phone remain quite usable even with 2 modules but with the third it is a mess.
With Firmware version 2.71.0.165 and 2.72.0.30 no way to work even with 2 modules attached. It only work properly only with one module.
I think it can be a problem of the CPU of the phone that cannot handle all that traffic. Consider that when the phone do not answer to NOTIFY messages the PBX start to resend it again and again till 20 times each.
This is like an avalanche effect for the phone i think.
We raised the T1TIMER on both the phone and the PBX to 5 seconds and this contribuited to let it work with 2 modules with firmware 2.70.0.150 (before raising the timer it was not working with 2 modules).
We tested more than one phone. We tested it with all POE Power and with power supplies attached and the result is the same.
I'd like to know if my theory is correct or there someghing that we can do.
I read on the documentation that are supported up to 6 expansion modules and wondering why I0m getting all this issues.
Thank you
Bye
Marcello