Windows startup can be mainly divided into three parts:
Of course there are many other possible factors which could make Windows to boot slow. Including hardware failures.
I strongly advise also to check the system and application event log (start
event viewer) and check for any errors. It might give indication about failing devices. For example if your HDD is about to fail it might cause delays by required read retries - this could delay startup a lot and decrease overall system performance.
Another approach you can try:
If your system seems to continuously access your hard drive or your CPU is always fully loaded it could have a big impact on overall performance too. Open the task manager and look for tasks which show a high CPU usage.
To get a more close overview about what's going on inside your system I recommend launching the
Resource Monitor. It will be able to show you CPU, Memory, Disk and Network activity per process. So it's easy to find processes which are causing a lot of system load.
In most cases processes which cause big Disk I/O load will have the most impact on system performance. In
Resource Monitor go to the Disk tab and sort the process list by "Total (B/sec)".
Also make sure to visit your hardware vendor update sites and update to the latest driver versions for all components.