That's an interesting limitation I didn't know. Though I haven't really relied on index-only scans much so far, the limitations regarding the visibility map seemed a bit too hard to judge to me. I assume that this works usually, but it's always a bit intimidating when a feature talks about limitations in DB internal data structures you didn't even know existed.

What surprises me here a bit is that this only provides a factor of two improvement. I would have expected the index to be much smaller than the table, though they include a bunch of columns in the index. At that ratio I'd be a bit worried about consuming a serious amount of space for that speedup, but that's impossible to judge without knowing the details. And if the performance is important enough here of course this is still worth it even if the index is large.