software - How many keywords are possible in Lightroom without a performance penalty?

Does anyone know whether there's an upper limit to the number of keywords possible in Lightroom without experiencing a lack in performance?

Answer

Your question isn't correctly written. I'll try to untangle it, tough.

You cannot say that till "n" keywords you won't get any performance degradation and from "n"+1 (ok, "n"+100 or whatever) you suddenly will feel it.

The performance degradation is a quite incremental process. It is rather an oblique line and not a stair-like graph. Also, of course, it depends on the hardware on which you're on.

That said, Lr organizes internally the keywords in several tables which are rather fat (wide - with many fields) in comparison with other programs thing which means that in some situations (queries) it will be (much) slower than other programs. I have practical experience with eg. XnView MP which is much faster in these things because it has just one table for keywords and this is almost a half wide (7 fields vs. 13 in Lightroom).

So, Rule #1: In Lightroom try to keep your total number of keywords at minimum. But if you cannot then there are other, better solutions.

Besides of the keywords table there is another table, the table of keywords assigned to each image.

This table is very narrow but, again, here Lr choose to have 3 fields instead of 2 (which is the minimum). Ok, one can argue that it is 33% increase but frankly because the 3rd field is an integer, I don't think that for 100.000-200.000 rows the performance degradation will be so big.

However, if the cardinality (the number of rows) grows, and this happens quickly if one assigns many keywords to each photo, then the difference in performance increases.

So, Rule #2: In Lightroom try to keep your number of keywords per image low. But if you cannot then there are better solutions, but the difference will show up only from several thousands images above.

Also, we have here a corollary:

Corollary #2a: If you cannot afford leaving Lightroom be sure to keep your catalogs small.

In conclusion we have Rule #3:

Rule #3: Lightroom wasn't designed for scalability in mind. If you want such a thing you can get other DAMs.

Disclaimer: While I tested other programs from performance POV (AfterShotPro is one of the best, Zoner has a hardcoded upper limit, ACDSee has a rather slow DB, like Lightroom), as I said, I personally use XnView MP together with Photoshop and/or Photivo (an awesome free RAW editor). It just happens to know well how XnView MP's DB backend is designed vs Lightroom and how is its performance curve. You can download it to see if it fits for you - it is freeware.