I totally understand the performance concerns, but the fact that the /tags page which can list 1000s of tags, already displays the tag descriptions via the title attribute.
For the sidebar tags though, they are significantly lower in numbers; I cannot imagine they’d affect performance more than the Tags page itself.
I can see how useful this can be, thanks for sharing @awesomerobot.
I think this is something we missed out on since a description for a tag is optional. We’re working on this at the moment and should have an update within this week.