Especially since a distributed DB with partitioned trees works wonderful.
Now if you have everything in one container, ummm yeah ANY directory will be sluggish.
Although eDIR has been shown to handle a Billion leaf objects in a single OU, doesn't mean you should.
Now if you want propaganda... sure there are tons of things out there comparing OpenLDAP to (MS-ADS, Netscapes thing, Sun's thing, IBM's thing...etc) But for every bad one they can show you... I'll have to bet you it'd be possible to should a bad one pertaining to the Proprietary one as well.
You have to rememeber distributed Directory is what it is all about. Unless you have a 64GB Memory monster and 8 processors... running in 64-bit mode (not well tested yet) and you have tweaked OpenLDAP to use all of it... and you don't have to worry about Disk IO...
IOW, if you use due diligience on the configs of the OpenLDAP servers and Client machines (being servers too) you should be fine.
Now, I remeber giving some advice on configs recently... are you sure the server is even being used properly?