Will dual booting Ubuntu and Windows on separate hard drives affect boot up time?

I was thinking of buying another hdd to run a separate linux environment on the same computer because I don't really want to repartition and use the space on my Windows boot drive. Will this slow the boot up time on Windows or create issues with Windows Boot Manager or GRUB?

There is no performance hit if the two operating systems are on separate disks, except the fact that you will get a boot menu when starting (although you can set your preferred OS and the time to wait for user input). Take good backups of the Windows disk before doing the installation, just in case, up to doing an image backup of the whole disk.

If you are new to Linux, I would advice to start with the Windows Subsystem for Linux (WSL), which does not require dual boot and lets you use both at the same time.

For more information see Frequently Asked Questions about Windows Subsystem for Linux.