The "how" really matters when deciding between microservices and a monolith. For instance, pumping out a bunch of Lambda/API Gateway web endpoints that back a static site served out of S3 or some other static hosting service would qualify as a microservices approach and yet is a huge time saver during the experimentation phase. No worrying about ops, monitoring or anything else off the list of overhead issues that isn't building your product. But if Microservices means configuring servers/clusters, service discovery and such, it actually adds to your overhead list and you're right that it will slow you down.

To my mind, services like Lambda are perfect for startups. In addition to scaling down, costing nothing when unused and allowing rapid iteration with zero operational concerns, they largely scale up and, in the unlikely event that the startup is wildly successful, they'll handle almost any load you throw at them until you've had time to re-engineer the backend to be more economical for the increased load.