Sorry but I have to vehemently disagree.
We are more or less "technicians" or at least "experts with some technical knowledge", and from the completely technical point of view, none of those tags (technical terms) means the same:
boot: "boot process involves all the phases involved on turn on the computer and get ready to use the operational system."
startup: "the system startup sequence after the OS' kernel is loaded. Contrast with boot, which covers what happens before the kernel is loaded."
as can be seen, from the technical point of view, the boot occurs first, and then the startup. They are intimately related, I agree, but they are completely different process. I can boot up my system without doing a startup and I can startup a system without needing the boot process, by restarting.
For the same motives, restart and reboot are different process. The later requires that the energy is drained of the system and that the POST process is executed, the former doesn't.
boot-time looks to me like it should be replaced with performance, since, whenever you are measuring the boot time of your system, you are doing it to profile or improve the performance during the process.
boot-failure seems like a mixture of boot problems and startup problems, it needs cleaning and remove the tag.
So, I agree that something is to be done with those tags, but merging them all isn't the solution. Cleaning and disambiguating seems like the actual solutions.