Will installing Ubuntu to dual boot with Windows slow down any OS or my laptop?

There are several factors to take into account but generally speaking, Ubuntu normally is at disadvantage when installing. This is because Windows is normally at the beginning of the disk, and Ubuntu (or any other distro) is at the end. Disks are faster then closer they are to the start, hence Windows will have shortest seek times, that will give it a boost opposed to Ubuntu.

Another thing is that Windows will not run at the same time that Ubuntu, nor the reverse. Hence while you are running Windows, only your system inherent performance is taken into account. I have not seen (nor imagine) any circumstance that Windows will slow down because you installed Ubuntu.

Bottom line, Windows normally will have the advantage on the hardware, but Ubuntu (Linux) will use it more efficiently. That's all.

Partitioning will decrease the current size of your Windows partition. This will affect storage capacity, not speed.

It would not slow down your machine because you're not firing it up all together :)

The fact that only one operating system will run in a dual-boot setup, hardware resources like CPU and memory is not shared on both Operating Systems (Windows and Linux) therefore making the operating system currently running use the maximum hardware specification. Regarding the disk issue, well unless the windows partition run out of free space which eventually can be fixed by removing some files.