Starting with SOLIDWORKS 2020, we are optimizing how we open older files to be more surgical about how we treat them.  You will find that files open as fast in SOLIDWORKS 2020 if they were converted or not.   

We also added an optional behavior to SOLIDWORKS 2020 to change how we save assemblies and drawings.  The default behavior is to convert every reference of the assembly or drawing to the current version when you save them.  This was existing behavior.  As you can imagine, the first save of a large assembly in a new version can take some time this way.  The new optional behavior is to save only the documents that have actually been modified.

The option is under External References -> Force referenced documents to save to current manage version.  It is on by default. 

Disabling this option allows save times in SW2020 to be the same whether or not the assembly was converted or not. 

Together, Open and Save times for the unconverted file become comparable with the converted file. 

When I talk to customers, the primary reason they run batch conversion tools is to improve performance.  With the new behaviors, do you still see the need to run a batch conversion tool?