strange battery and processor behaviors

Sorry for the length of this note but I want to be thorough.

I have a early 2020 MacBook Air with an Intel i3 motherboard. It was on the shelf for a while. I gave it to my daughter when I bought myself a MacBook Pro and she used it so rarely that she just gave it back to me about six weeks ago. She has to use work computer so this one basically never got turned on. When I turned it on using AC power, I noticed that it began to run like molasses. Very slow. I also noticed that if I pulled the AC, it would shut off what immediately. So I charged the battery and saw that the battery itself could get charged to 100% but it would not sustain the computer at all. So I thought well maybe I need a new battery and what the heck maybe a new motherboard. So I put in a different compatible motherboard using I -5processor and a new battery. I’m very good at installing these things and everything was installed perfectly and when I turn the computer on it, installed the new version of Mac iOS and I know that everything worked speakers worked. Bluetooth works Wi-Fi work the fingerprint   touchpad worked. However, it ran like molasses, loading so slowly and even though this battery also charged to 100% percent the computer only runs in AC and if I try to start the computer on battery or unplug the AC the computer shuts off.

I saw my questions are does anybody else have same experiences and what else about this machine could be causing this kind of aberrant behavior in the battery and in the speed of the computer other than the battery and the motherboard.? Any   opinions , suggestions theories and opinions   would be welcome.