If you consider development to be releasing features and stop timing after launch, then yes, it could slow down the development process. But if you consider development to be releasing customer value and include the time needed to fix issues after launch, then no, it definitely does not slow down your development time; in fact, it speeds it up.

Customer research helps to ensure you are building the right thing, but also building it right. If a team is releasing features without customer research, there is a high risk that changes will need to be made after launch or, even worse, that no one will see the value in the feature at all.

Another way to look at customer research is as a risk mitigation process. Generative research helps to ensure you understand the real job that customers are hiring your product to solve, and evaluative research helps uncover any potential issues customers might face using the product. It moves the time spent from the end of the development process (fixing issues) to the beginning.