Cleantalk causing slow database issues, wpsy_cleantalk_sessions table

We noticed recently that our website (videogameperfection.com) was running slowly. We asked a professional to take a look at it.

He told us that “wpsy_cleantalk_sessions table has over 13,000 entries and lacks a primary key on id, resulting in slow (~1.2s) lookups.”

What is this table, how do I empty/optimize it and prevent this from happening?

Thanks,

Matt