How are you measuring the "slowness" ?
After restarting, open the Windows Task Manager,
or run the "resmon.exe" Resource Manager.
It could be Windows Update checking for updates,
monopolizing your CPU & your Internet.
It could be Disk Defragmenter, monopolizing your disk-drive.
It could be your anti-virus software, scanning your "C:" drive.
Not all of these background tests are launched every time that you restart Windows, as you have reported.
Do you have an older computer, with a "spinning" disk-drive, that is much slower than a newer computer with a SSD (solid state device) or NVMe (next-generation SSD) ?
I have a new PC with NVMe SSD where is installed the operative system and an HDD where I have only data. I have tryed to open the Windows Task Manager when my Internet connection is slow and I've noticed nothing strange, no process were consuming RAM or CPU. When I turn on my pc and internet is slow, is slow only Internet, everything else works well and the pc is fast as usual