Server side improvements
Now there is more than a ton of improved things, including increased SNMP trapper file size, optimized server-proxy data exchange protocol, changes in default data storage period, more efficient sync of configuration cache and many more. All the improvements can be studied in documentation.
For example, here you can see how significantly cache locking time during configuration synchronization drops down at the point of 13:35, when Zabbix 3.4 comes into action. No more wait while sync is in process, just carry on with your tasks.
In the new version, the old mechanism is dropped. Instead there is an new option in action configuration, which allows to pause notifications in the host maintenance phase if you wish so.