I'm talking from personal experience. Your ability to keep a train of thought is impaired, you struggle to build the house of cards in-memory to do any kind of extended analysis, the cards you put down simply vanish from memory. You feel empirically dumber.

At its worse, you start to talk about a topic, branch into a subtopic, and are unable to find your way back. You find yourself asking people "what was I talking about?" all of the time, imagine doing this in the middle of a presentation or while talking to a client.

Relevant Study: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3931635/