Speeding up SQL query with multiple joins

Question

I have a .NET e-commerce solution running off a mid-sized SQL Server express database. The system queries the order data which involves many joins (potentially 20 tables) which is quite slow, particularly during periods of heavy use, and I think I have exhausted the options for indexing the tables and optimising the queries.

I now believe the best option going forward is denormalization - see https://msdn.microsoft.com/en-us/library/cc505841.aspx

What I would like to know is:

1. Would SQL Server columnstore indexes be a better option?
2. I am considering using in-memory OLTP on the denormalized tables because having the data in memory will undoubtedly make queries faster but it doesn't seem like the intended use, so should I?
3. Should I use something like ElasticSearch instead, and what would be the benefit over SQL Server in-memory OLTP?
4. Should I use SQL Server OLAP instead? Seems like overkill...