Improve sql server lookup table performance

We have a sql server database (on azure) with one large table c1billion rows. Each row consists of a key char32 and a value int. All queries select multiple values from the table using a key in clause. Sometimes we have 1000+ items in the in clause and the query can be very slow. What are the top things we can do to improve query performance in terms of table structure (partitioning?) and query structure (maybe chunking queries)?