Does integrated graphics slow down CPU?

Integrated graphics is a separate chip from the CPU, but it is in the same package.

As such the GPU does not "use" CPU time to work.

That does not mean that the integrated GPU doesn't slow the CPU down slightly though. The integrated GPU uses the same memory as your CPU, as a result graphical tasks can take bandwidth from the CPU and cause some level of contention.

Given the massive bandwidth of modern memory the penalty will, for tasks such as desktop composition and "normal" programs, be of the order of a few percent of memory bandwidth. Small enough that you will almost certainly not notice. The Windows desktop compositor is quite efficient and only updates when graphics has changed leaving a smaller amount of ongoing bandwidth required for the GPU to put the screen out to your monitor.

For gaming and 3D modelling and other graphically intensive applications there may be a lot more bandwidth used by the GPU to deal with textures, shader programs, geometry and so on over several passes. That would use a lot more bandwidth and you will reach a point where your CPU and GPU are fighting over bandwidth to do all the work to show graphics and handle all the other system tasks. That would be where you definitely want to switch to dedicated graphics. Light games might be fine though.

Disabling integrated graphics and using a dedicated card will remove system memory contention.