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.
Thanks @ivanviso, nice small project. Due to heat dissipation issues I could only use 2 out of 4 cores on my old smartphone but I'll give this a try and see if I can use all 4 now!
I'm also using two Galaxy S3 to crunch.
Do you know if there's a way to also remove the battery? By connecting the phones power directly with an usb charger or so?
It depends on the phone, some work just fine if you remove the battery while they are on, some can be made to run like that just by moving the pins that run to the input of the battery to the output of the battery, some can be made with a small soldering, and some are not able to be modded.
I found this : https://forum.xda-developers.com/galaxy-s3/accessories/how-to-samsung-galaxy-s-iii-battery-t3417303
Thats the good thing about popular phone models :)
Or if you have cooling paste for cpu's laying about try putting that on the chipset and put a few copper chunks (or a few stacked eurocents) on it, metal is a good heat conduit.
Wish I would have read this before I ballooned my phone from heating, lol. Although it still does work so I may need to take your advice and make it a little more robust to prevent it from going fully dead. Thanks for the tip @ivanviso
Please be very careful, a battery like that may start burning any time. Should be no problem as long as it is kept far from any combustible object.