Sounds like replacing that mobo would be kind of a hard job, and many other upgrade options seem like they would be super expensive. One even ran a bit more smoothly on my freakin school laptop. I have noticed that when I watch people play certain PC games, they seem to run quite a bit more smoothly on their PCs than on mine. It must have been one of the cheapest new PCs my parents could find. I didn't realize my PC was that bad! I thought it was a pretty high end one but I'm probably remembering wrong. No other settings change would likely give you better performance. That, IMO, is about the absolute best you could do to increase performance. Unfortuantely, recent AMD GPUs are sort of hard to come by MSRP now thanks to cryptocurrency miners snatching them all up, but just about any AMD GPU since the HD 5xxx's would work well for Dolphin. Just about anything from the last 6 or 7 years now should work just fine, but I would suggest maybe the GTX 1050 (about $120-$150, depending on what manufacturer you buy from), which will work wonderfully for Dolphin at higher graphical settings. The next choice is to get a dedicated GPU. I can tell you first hand the x4 760k is super cheap, I bought mine off ebay for $30 total, so that's good. That would improve performance to a point, but would also require you to buy a dedicated GPU. I actually had to do this for my work desktop, before I swapped the mobo entirely, which definitely improved performance over the APU I had.

Unfortunately, your mobo uses the FM2 Socket, meaning your only choice to upgrade (to something remotely usable) would be to the Athlon X4 760k. What this means, essentially, is that no matter the settings (or even getting a dedicated GPU, as some others suggested) Dolphin will run like ass for you. Combined, you have just about one of the worse setups for Dolphin. Ryzen improves this, making them better for emulation). APUs have notoriously awful CPUs, for one, and AMD CPUs also have relatively low IPCs (or, at least, their older ones. TotalInsanity4 and Jdbye are correct, the APU is the issue here.

Dolphin emulator mario kart wii slow