What are the benefits of moving to a .NET5 Optimizely implementation?

The answer to this question will depend on the type of user you are. On the short term, the visitors will enjoy the same great experience, but with faster page rendering times, which is a good thing since visitors are not willing to wait.  The way the Optimizely team proceeded with the migration of the platform make this migration seamless to the visitor experience.  The migration was done so that most of the functionalities rendered on the site would not change.  Visitors are expected to be delivered the same experience.  

For the content editors and site administrators, they will notice a faster and more unified interface.  The content authoring experience was updated to provide smoother and more efficient workflows.  With all the acquisition of the last few years (Idio, Optimizely, Zaius, Welcome), the Optimizely product team had to rework a little bit the editor and admin interface so that the experience would be more seamless between the products.

It's the developer that will see the most benefits.  They will be able to take advantage of a modern development platform, aligned with the latest Microsoft platform, standards and patterns.  Developer will also experience faster build and start up time, which will provide a huge time saver.  This migration will also allow for a better headless experience, allowing teams to use the content with other technology stack, be it mobile of more modern front-end framework.  This will help provide a greater multi-channel experience.  

And on the long-term, migration between the .NET frameworks will be more predictable and should be easier to plan and handle.  We already see the first migration coming in the next few weeks.  Since the availability of the .NET5 implementation of the Optimizely products, .NET6 was release by Microsoft.  The product teams are already working on a migration plan, which will be a lot smoother since the change between the .NET versions are smaller.  And there is already packages that are available on .NET6.

So, the general benefits of this migration are a faster, modern platform that is aligned with the latest technology stacks and ready for future development.