[2020] Speed Acceleration not working anymore

Since your other city is fine, my guess is it's the normal slowdown that happens as a city grows and has so many citizens, that the simulation slows down to be able to keep up with all the calculations. It happens for all cities and computers at some point. The more powerful the CPU, the larger a city can be before the speed up options stop having a real effect.

Most of the CPU usage comes from calculating where people go. If you use any mods that make the AI smarter (like TMPE or Real Time), removing those might increase your performance some. But the slowdown will happen at some point regardless - just at a larger population instead.

You always remove the trees to see if it makes a difference. Just save first, then remove them with a mod like Move It or Better Bulldoze and see if that helps performance.