Why does Windows 10 take so long to install cumulative updates?

It can be anything. Many things are involved here. Your speed of your system, which storages you uses, how much free space you have on your C-drive, how much memory is installed into your system. If like 4Gb than swapping can occur and that is a real killer how long actions are taking. It can extend an update by hours.... , how fast connection you have with the Internet. It also depends on what time you initiate a update. Sometimes when the US is awake things are slower then inside the EU. MS has a farm of servers that upload stuff all the time. (It's busy as milions sees a new update and must upload the update) Capacity is an issue.

In order to see what your system is doing; Start a powershell as admin and issue the command 
"Get-Content C:\Windows\Logs\CBS\CBS.log -tail 10 -wait" You see the internal CBS engine busy with the updates. It is a flow of installments and checks. You can also inspect the CBS.log file when finished (C:\Windows\Logs\CBS\CBS.log) Every action has a datestamp. So check if it's taking hours at what moment you see your system waiting for many minutes until it is finished. I have to warn you; the messages are quite technical of nature. but fishing a few line might help to see why your system is so slow at updates.

To prevent any disturbances during an update it is wise before an update (if it is released but not offered to you) to run the commands "SFC /scannow" and "DISM /Online /Cleanup-Image /RestoreHealth" (as mentioned earlier) inside a CMD-box as admin. These commands checks the integrity of Windows and repairs things if they are faulty. If they are faulty during the update the CBS-engine has to spend much time to repair things first before it can start any update. It's wise to check the integrity once every month. SFC /scannow will check your system quickly if there are faults. If so it repairs it automatically. DISM /RestoreHealth checks the integrity of your WinSxS dir where all the ComponentStore packages are residing. It also checks it and if faulty downloads the package from MS in order to repair them automatically.