How to improve speed or performance of Linux Mint XFCE on old computer

Decrease swappiness whereby the use of swap is curtailed
$ cat /proc/sys/vm/swappiness <enter>
returns the value of swappiness which is 60 initially. Make this 20 as follows:
$ sudo xed /etc/sysctl.conf <enter>
Add a line to the end of the file
Save the file.

Disable Java in Libreoffice
Open Libreoffice Writer.
Go to Tools -> Options -> Advanced -> Java Options
Uncheck Java Runtime Environment.

Turn off unwanted startup applications in Session and Startup -> Application Autostart
Uncheck the below in the start up applications
a. System Reports
b. Warpinator
c. mintWelcome

Remove unwanted fonts
$ sudo apt-get remove "fonts-kacst" "fonts-khmeros" fonts-lklug-sinhala fonts-guru-extra "fonts-nanum" fonts-noto-cjk "fonts-takao" fonts-tibetan-machine fonts-lao fonts-sil-padauk fonts-sil-abyssinica fonts-beng-extra fonts-gargi fonts-gubbi fonts-gujr-extra fonts-kalapi "fonts-samyak" fonts-navilu fonts-nakula fonts-orya-extra fonts-pagul fonts-sarai "fonts-telu" "fonts-smc*" fonts-deva-extra fonts-sahadeva <enter>

$ sudo dpkg-reconfigure fontconfig <enter>

Remove flatpak completely
$ sudo apt-get purge flatpak <enter>

Remove mlocate to reduce the disk search & seek time.
$ sudo apt-get purge mlocate <enter>

Improve display performance if using XFCE desktop
In Settings -> Desktop Settings replace Xfwm4 + Compositing to Xfwm4-Compton.

Remove compiz for good as it is not of much use
$ sudo apt-get remove compiz-core <enter>

Tweak Firefox for performance
Start up firefox and in the URL box enter about:config. Accept the risk. Refer this post.

Restart firefox and see the improvement.

Speed up boot time on Linux
Refer the post on how to speed up boot time on linux too.

Periodically go to about:support in Firefox. Look for Places Database and click on Verify Integrity to reduce firefox bloat.

Lastly, never use applications like bleachbit to clean or optimize.