Entity Framework being slow isn't the problem with it. The performance of an ORM only really depend on 2 things. The time it takes to do the mapping, which is trivialized by the actual queries and is really a micro-optimization, and the SQL generated.
The later is fine in almost all of the mainstream ORMs, and the only real issues come from N+1 queries, and not aggregating queries that can be by using the context and futures. ActiveRecord based ORMs have issues because of the way context vs model objects is managed but they can still handle it fine, the N+1 problem is almost always a developer thing.
The reason Entity Framework sucks is that its API is poorly documented, so short of reading the code, no one knows how to do things correctly, a lot of its features are not exposed through the tools, thus encouraging using the RAD/shitty ways of doing things, and they pulled it in all directions trying to make everyone happy, ending up with a lowest common denominator.
You can get perfectly good performance out of EF, it has all the features an ORM needs to achieve it. Its just hidden/awkward/changes all the time/forces you into writing shitty code.