Hi there,
I'd love to know what you expect. You are multiplying the number of visible/reflected instances by 5 and the rendering time is about 5 times slower. That seems pretty expected to me.
Then you have 2 diffuse bounces and your volume material has some scattering component despite being a strong emitter. So you get multiple scattering in the volumes (and in the reflections on the ground) which is VERY expensive.
You can try to set the Diffuse Depth of your volume material to 0. While you will lose some energy, indirect illumination contribution on a pure emissive volume is rather low on the final render but the render times is very high.
Use the AOV diagnostics_ray-count you will see where you rays goes specially that you are using adaptive anti-aliasing. BTW, set your AA filter to Gaussian 2.0 2.0 to speed things up slighty instead of Blackman-Harris 3.0 3.0.
On the left Diffuse Depth on the volume material is set to 0 and on the right it is left by default (2 here)
On the left, we can clearly see that the most of the rays are spent on the ground. However on the right, most of the rays are spent on the fire. So the more fire you have on screen the slower the render gets.