High CPU during calendar-sync of patient application to exchange

Hello everyone.
A customer (hospital) has a software with which the patient appointments with the respective doctors are planned.
Every 30 minutes this software synchronizes the calendar entries in the mailbox of the respective doctors with a service account with impersonation rights via EWS.
Normally the CPU utilization of the Exchange Server is about 20%. During the 2 minutes that the calendar synchronization takes place, the CPU usage increases to about 95%. During these 2 minutes the users have a freeze in Outlook.

It is the w3wp.exe-EWS process and Exchange.Store.Worker.exe that increases CPU usage

I have already tried with Throtteling Policy, but without success so far. (But I don't really know much about Throtteling)

EwsCostThreshold : 50
EwsMaxConcurrency : 27
EwsMaxBurst : 300000
EwsRechargeRate : 1800000
EwsCutoffBalance : 0
EwsMaxSubscriptions : 100

Does anybody have a tip what else I could do?
Thanks and best regards

Marc