I came across two URL links about mirrors and the mention of one of its limitations. Said limitation is the fact that the viewer will show the effect of only one mirror reflection probe at a time of the mirror closest to your camera. The two links are provided below.
The limitation from what I've read in those two links is because of possible performance issues. From the wording it doesn't appear to be a sim server performance issue but more about the performance of the computer being used by the user.
If the performance issue is with the server which I doubt because of the chosen words in the provided links above, then I suppose my feature suggestion will have to wait a few or several years for the servers to improve before possibly being implemented.
However if it's because of the computer being used by a user, then you should at least give them a choice on whether or not to allow two or more mirror reflection probes to be seen appropriately for whichever ones are closest to your camera. The option for this can be applied to the Preferences - Graphics area in the viewer with the label being something along the lines of "Mirror Probe Count" or "Max Mirror Probes" or whatever you feel best to label it as.
As a side note, I've tested my viewer with the highest graphics settings with no stuttering and no slow down when everything is loaded but that is considering the average sim. This can be another story if the sim in question has a lot of content that might end up being heavy on the computer but hard to say until it's fully loaded then it might go smooth or slow down. The motherboard, ram (16GB), and CPU (AMD 6 core 3.3GHz) on my current computer was bought back in 2010. The the boot drive is a SSD, while the graphics card is a 8GB Nvidia GeForce RTX 3050, both of which with consideration of time scale, were "recent buys". Meaning that if I have a computer this old with only a few new parts with no major issues running SL, then adding the ability to see more than one mirror reflection probe appropriately shouldn't be an issue for computers like mine. Though as mentioned, at least give people the choice to decide if their current hardware can handle more than one mirror probe being rendered appropriately.