The tabular representation of the new features and improvements related to performance since Zabbix 1.8 can be found in the online chapter, Performance Features and Improvements since Zabbix 1.8, which is available at https://www.packtpub.com/sites/default/files/downloads/B03410_Appendix.pdf.
According to it, the Zabbix SIA guys are working hard on every Zabbix version to improve performance. I mean, they are thinking about performance and are creating ways to improve it. And how can we stay updated with the enhancements and new features of Zabbix? There are several ways, all of them accessible by and available for any user. The main points of reference are as follows:
Zabbix's official forum: https://www.zabbix.com/forum/
Zabbix documentation: https://www.zabbix.com/documentation/2.4/manual/introduction/whatsnew240
Zabbix webinars: http://www.zabbix.com/webinars.php