When a computer signs in, it does not just load the desktop. It also launches everything that has asked for permission to start automatically — cloud sync tools, chat apps, updaters, printer utilities and games launchers.
Each of those wants disk access and memory at the same moment, so the first two to five minutes after login are the busiest the machine ever gets. That is why a computer can feel unusable at first and then be perfectly fine.
Most of these apps do not need to start early. Sync tools can wait, and updaters can run once a week. Removing them from start-up does not uninstall anything; it simply delays the launch until you open it.
The gain is largest on machines with a spinning hard drive or limited memory, where competing for the same resource turns a ten-second task into a two-minute one.