As @discerningLikes said. Ethereum is too complex to scale. Maybe it can in the future, but not currently. Miners can't raise gas limit because of too many orphans resulting from it.
I thought the high orphan (uncle) rate was related to the fast block speed making it difficult to propagate blocks fast enough.
The Ethereum Virtual Machine is very slow. I think that is what's the bottleneck in the validation process.
That's also true but if you raise the block time you will just need to have more transactions which result the same. I think it's the validation that's too slow and is the bottleneck.
