What to do if Windows 10 does not run smoothly on the notebook?

I installed Windows 10 on my girlfriend's notebook. Before it was Windows 7. Unfortunately Windows 10 does not run smoothly. The calculator is very slow. I don't want to buy a new notebook. What can I do? Back to Windows 7, although there will be no updates from next week?

Have you tried reinstalling Windows 10? I don't trust the upgrade system and a new installation can work wonders.

If that doesn't help, you can't do much more, at least not until you know the reason.
Is e.g. Too little RAM installed and expandable can be repaired with relatively little money.
An SSD instead of an HDD can work wonders, but in Windows itself you can only notice this at startup and not in general operation.
If the CPU is too slow, you can do practically nothing, very few notebooks allow you to install a new CPU.
If the notebook gets too hot, the CPU reduces the clock speed to protect itself. In this case, it helps to ensure that air can cool all around. In addition - especially with old notebooks - it can also help to clean the inside of the device and to replace the thermal paste.

Most of these points require some skill, so you should look for help. If you want to do it yourself, check out online instructions for exactly this model and get suitable tools beforehand.

Of course, I can't recommend Windows 7 for the reason you mentioned, but if a new notebook is an option in the near future, I see Windows 7 as a suitable interim solution.

For private lame Windows installations, I replace them with the free Linux Mint - the computer then remains permanently fast, is well equipped and virus-free without any power-consuming helper programs.

Before doing such an action, "you" usually make a backup - I prefer Clonezilla Image Backup.

If, after installation, as in your case, the expectations are not met, you could restore the old state without any problems.

Unfortunately, you do not write anything about the notebook he is dealing with here; if this is not technically up to date with the requirements of Windows 10, it can't be anything.

Unfortunately, it is not technically up to date.

Yes, but some programs do not exist for Linux.

With Wine, around 10,000 Windows programs can be used under Linux,

Windows on Linux can also be used with Play on Linux

there are lists with correspondences of Windows software → Linux software with the same function

In the repository, i.e. The AppShop, of every Linux there are 10,000 free programs that you should first look through - with Linux Mint there's the "Start" button and the 2nd icon from above, the "Application management"

If one day I encounter an indispensable Windows program, I'm looking for my old hard drive, there's still a Windows on it.

This is also probably the reason why newcomers are advised to do a dual boot installation, so leave the old, often slow, Windows on the disk and choose "Install Linux next to the old system" for the Linux installation. The rest will then run automatically. After that, each time you start the computer, you can choose whether Windows should run or Linux (Linux starts automatically if nothing is selected). At some point the realization comes that Win will never be started (as with me), because Linux offers everything free of charge that is needed in a nerd's everyday life. In future installations, Windows no longer needs to be dragged along when the more user-friendly Linux is installed.

I have to think about it.