The answer to this depends on the type of filter you're using and your existing flow. Some of them may have a slightly negative effect on water pressure, but this should be hardly noticeable. There are some filters out there that can even improve the water flow. There some tricks to be able to get around this, such as clog prevention. If you already have a powerful level of flow, then you don’t need to be worried. If your flow is poor, then it’s best to double-check you’re getting a filter that won’t negatively affect it.