At this point I think 4GB is really the minimum system memory I'd recommend for a Win 10 system. I can't remember what MS says but I had a laptop with less and I just gave it away because I couldn't justify selling it. So memory could very well be your limitation. Something that might help would be an SSD. I've had good results putting a cheap SSD into several people's systems with only 4GB of memory. It's not a miracle but when memory is limited you end up hitting storage more often so faster storage tends to help.
In answer to your question, yes memory is usually a motherboard limit. There are limits to what a CPU can address but usually you end up limited by the number of slots and the capacity that can be installed.
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