I cant install old nvidia driver after kernel update

You have not given the details on How you tried to install, why you were not able to install (error message, etc), what exactly is the difference in performance that you noticed. It is always a good practice to provide as much (relevant) info as possible. It will help you get a quick & proper solution (or) at least, more closer options - besides helping/motivating the people who are trying to help.

If the driver is showing as the Recommended driver under Activities -> Additional Drivers, then it the latest stable legacy driver that supports your Ubuntu version & kernel version - according to Ubuntu.

Judging by the version number, the one that is installed now, seems to be the more later (if not latest) subversion of Nvidia 390 driver family. However, at times latest kernels & latest drivers don't go hand in hand.

You can install a particular version of NVIDIA driver by downloading it from official NVIDIA site (link below).

https://www.nvidia.com/Download/driverResults.aspx/137276/en-us

Note: This is for 64-bit GPU. If you have 32-bit, browse back in the site & select the appropriate one.

Now, before installing the particular version, you need to remove the currently installed version(s). Follow the link below for instructions.

https://askubuntu.com/questions/1097433/ubuntu-18-10-how-can-i-install-a-specific-nvidia-drivers-version

The download is a file with .run extension, and can installed by using just sh command (like below).

sh NVIDIA-Linux-x86_64-304.125.run