Why does my computer take so long to start up?
Boot time divides into firmware initialisation, operating system loading, and everything that runs automatically once you reach the desktop. The third is usually the largest and the easiest to improve.
Startup programs accumulate silently. Most applications add themselves without asking, and a machine used for a few years can be launching a dozen or more before you touch anything. Both major operating systems provide a startup manager showing what runs and, helpfully, the measured impact of each. Disabling anything you do not need immediately at login makes an immediate difference, and nothing there is required for the system itself to function.
Storage type is the single biggest hardware factor. A machine still booting from a mechanical hard drive will be dramatically slower than one using solid state storage, and no amount of software tuning closes that gap. If the machine is otherwise adequate, this is by a wide margin the most effective upgrade available.
A nearly full disk slows everything, including boot, because the system needs working space. Keeping a reasonable margin free is worth doing for performance rather than only for capacity.
Pending updates cause slow starts that resolve themselves. A machine that installs updates during shutdown or startup will occasionally take much longer, and this is expected rather than a fault.
Firmware settings occasionally contribute. Fast startup options skip some hardware checks, and a long delay before any operating system logo appears usually points at firmware waiting on a device, often a network boot attempt or an unresponsive drive, which can be disabled if unused.
Measuring before and after is worth doing, since perceived boot time and actual boot time frequently differ.