What is an allternatve approach to many fields in an entity? I have an entity bundle 'A' that has about 30 fields in it.
This is probably affecting the page load performance of any view requiring to load these entities.

As an alternative, I was thinking about putting a number of the entity fields in a separate entity bundle 'B' which is connects the entities to bundle 'A' with an entity referenc field.

Would that improve performance or is there a better solution.
Like Clive said, testing would be the definitive answer. But if you look at the mysql queries that will have to be executed behind the scenes, I'm pretty sure you'd find that your bundle A with 30 fields will be simpler and faster than the bundle B option. You can use the Views Show Query module to output your view queries in both scenarios and compare them. In bundle B option, I'm pretty sure the mysql query will have to not only look up all of A's and B's fields but also look up the relationship between A and B bundles, an expensive additional JOIN.