An easy, but very helpful tip : If you have an old phone you no longer use crunching GRC, remove the casing to improve cooling, and with that, perfomance.

Phone should look like this :

The ideal is to use one of those to remove the screws :

But a small knife works just as good.

In this case, a very old quad core phone, it is more than enough with removing it's casing to keep it cool. Owners of more powerful devices, eight core phones and the like may want to install a heatsink like those :

(they are very cheap and easy to buy in many locations : https://www.sparkfun.com/products/14340)

Or even one of those, if you feel the need : https://nl.aliexpress.com/item/Gdstime-5-V-USB-Fan-92mm-92x92x25mm-PC-CPU-Cooling-Cooler/32834660864.html

A simple option is to use aluminum tape :

Also very cheap, you should be able to get a cylinder for $5.

Or even make your own dissipator if you are crafty enough.

You could use scissors to cut some soda cans in half, stack them, then bend them (very careful with not cutting yourself) and safe it to the phone with tape or glue.