VDA optimization is by default included on VDA upgrades. Users will not be able to uncheck the optimization options on VDA upgrade wizard. Refer CTX125874 to understand the Specific Optimizations Performed by the Virtual Delivery Agent 7.x Installer​.
 
VDA Upgrade wizard does not list the option Features on which we can select/deselect ‘Optimize performance’. It skips everything before and starts with Firewall tab in the Wizard.

 

 
As part of the optimization, the Maximum Event Log size on the VM will be reduced to 1.028MB (1028KB) from the default size.



Solution:

Those who have set additional size for event logs and need these logs on the server for future investigations should back up event logs on the VM before starting the VDA upgrade on it. Since optimization of performance is forcibly applied irrespective of the user's intention, user data such as event logs disappear by the amount reduced by optimization.

Note: For example, If a user set their maximum size of the event logs to 1 GB, then after the VDA upgrade it will change to 1028 KB. Because of this, the event log data of about 999 MB is lost. There is no means to restore the logs of the differential capacity which will disappear due to the reduction of the event log file size, unless there is a backup. Hence it is recommended to take a backup of the logs before upgrading VDA.

You can resize the Event log file size to the desired value manually after the upgrade and restore the backed up logs. (https://technet.microsoft.com/en-us/library/cc748849(v=ws.11).aspx)

Workaround:
There is a command line switch that controls this option so that you can avoid the optimizations in an upgrade scenario, which is to do command line upgrade using  “XenDesktopVdaSetup.exe  /quiet  /components  vda”, with expected behavior that TargetOSOptimizer.exe will not being called by installer and the maximum event log size does not get reduced.