When Windows starts, if you have network drives mapped then the startup process interrogates all of those network drives to get current folder/file information for each of them (since they will become part of what is shown when you open Windows Explorer). The more mapped network drives you have, the longer this startup time interrogation process will obviously take.
Depending on the number of folders/files on those network drives, and depending on whether the drives are even spinning (or might have been spun-down from inactivity, and now must spin-up for the folder/file interrogation), this might take a while. Are they FAT, FAT32, or NTFS? This too affects the speed with which the folder/file interrogation completes.
Also, do you have 10/100 network speed, or gigabit 10/100/1000 speed? This makes a huge difference in how long the network drive interrogation process takes. Remember, it's not just the NIC and router speed here... if you don't have proper ethernet cables you're getting 10/100 forced.
There's no way to avoid this "overhead" or to make it run faster, if you want the convenience of being able to quickly and easily access any folders/files on specific network drives with the same speed you can access your local machine drives. If you "disconnect" so that these are not always pre-interrogated at Windows startup time, you can obviously speed things up significantly as you've described. But then you'll still have to connect to them on-demand during your Windows session, and you'll have to go through that same overhead later rather than earlier.
I suppose keeping the number of mapped network drives to a reasonable minimum will minimize the startup-time overhead. But for the ultimate convenience of having your network drives as easily and quickly accessible as local drives, I'll gladly let an extra minute or two go by at Windows startup.
