I'm having the same problem right now, installing multiple Ubuntu flavors and derivatives on a mid-2009 MacBook Pro 13-inch (5,5). I started with Ubuntu Studio 22.04 because that's what I wanted first. I tried Kubuntu afterwards (Studio is based on Kubuntu), suspecting that it's a problem with the low-latency kernel. I also tried mainline GNOME Ubuntu, Pop OS 22.04 (the NVIDIA version), and Linux Mint Cinnamon 20.3. Settled on Linux Mint XFCE 21 for now because it is the first distro where I can actually keep using the computer without having to hard-shutdown by waiting like three minutes for Firefox to close.
In all of the above, the computer starts to freeze when I use Firefox or Thunderbird for more than a minute while connected to the Internet, and it's due to the CPU maxing out, according to the widgets I put on the panel of each of the DEs. On mainline GNOME Ubuntu this also happened by merely opening the LibreOffice app (I could successfully open and use LibreOffice in all other cases). I have 8 GB of RAM and never passed 4 GB while trying to make things work, so I don't believe RAM is the issue.
I got a brand new SSD to clean-install, and completely wiped all the data every time I tried a new distro. All Ubuntu versions were 22.04, as well as Ubuntu Studio 21.10 to try to address a separate Wi-Fi issue -- and I USB-tethered to my smartphone to get online. (I couldn't try Lubuntu because my physical trackpad clicker isn't working, so I'm limited to distros which either have tap-to-click enabled out of the box, or I can use the keyboard to go to settings and enable it.)
I really wonder what it could be, because I'm typing this now on a decade-old HP Mini with a much less capable, absolute potato of a CPU, maxing out all the time -- 9 tabs open right now on Firefox Snap -- but it never freezes up to the point of un-usability and hard-shutdowns because of that. Lubuntu 22.04.
In all of the above, the computer starts to freeze when I use Firefox or Thunderbird for more than a minute while connected to the Internet, and it's due to the CPU maxing out, according to the widgets I put on the panel of each of the DEs. On mainline GNOME Ubuntu this also happened by merely opening the LibreOffice app (I could successfully open and use LibreOffice in all other cases). I have 8 GB of RAM and never passed 4 GB while trying to make things work, so I don't believe RAM is the issue.
I got a brand new SSD to clean-install, and completely wiped all the data every time I tried a new distro. All Ubuntu versions were 22.04, as well as Ubuntu Studio 21.10 to try to address a separate Wi-Fi issue -- and I USB-tethered to my smartphone to get online. (I couldn't try Lubuntu because my physical trackpad clicker isn't working, so I'm limited to distros which either have tap-to-click enabled out of the box, or I can use the keyboard to go to settings and enable it.)
I really wonder what it could be, because I'm typing this now on a decade-old HP Mini with a much less capable, absolute potato of a CPU, maxing out all the time -- 9 tabs open right now on Firefox Snap -- but it never freezes up to the point of un-usability and hard-shutdowns because of that. Lubuntu 22.04.