Earlier quoted context omitted.
OpenLDAP can also handle millions of accounts, is open-source, supports awesome replication and availability topologies and doesn't require any consultants or otherwise bullshit dependencies/support contracts. I'm sorry but there isn't a commercial product that can compete with OpenLDAP and there may never be because it is just that good if you know how to use it. (yes, the learning curve is high, that is the only cr…
Here's a little known fact for you: OpenLDAP is a commercial software made by a company called symas. https://symas.com/ The open-source edition is only a facade with highly stripped features. If you want any decent replication, HA, performances or bugfix. You have to pay for their OpenLDAP gold edition, which last I checked was $100k for a site license.
If you don't pay Symas money then no, you won't get support, or bugfixes developed for you.
However I doubt there is a large performance difference, it's already very efficient so it would be in the single percentage points.
As for replication you can do almost anything with the open-source version by combining syncrepl/delta-syncrepl into the topology you want, hell you can do N-way multi-master if you wanted.
I'm not sure what you have against using open-source OpenLDAP, maybe you should try it first?