Forum OpenACS Q&A: Response to Article on ACS x Zope
The AOLserver/Tcl/RDBMS solution has proven to be immensely scalable, whether or not the ACS is involved. That's one of the appeals. This is largely due to AOLserver being fiendishly efficient and due to the database drivers (for Oracle and PG, at least) and the Tcl interpreter being built-in. And with the fact that the folks trying to keep AOL up and running have put in many, many hours making sure that the threaded Tcl interpreter works very well.
Zope appears to be untested at the high end. I doubt if there's anything intrinsic in the architecture that would require it to be inefficient, though. If there are scalability problems I'd guess it would have to do with the fact that the AOLserver folks have HAD to make certain the server can scale, because AOL and Digital City are two of the busiest sites in the world, while the Zope folks haven't dealt with sites of this size. (100,000 hits a day ain't nothin').
If there are scalability problems with Zope, my guess is they'll fix 'em before you really run into them, unless you're planning to roll out Amazon The Next Generation or something in the next few weeks.
Since you mention e-commerce, that particular module in the ACS is apparently pretty easy to modify. In particular, it's one that's been templated so you can modify the look without digging into acres of Tcl code. And it's designed to be largely configurable from the web admin pages. I've not worked with it myself, though I shortly will be setting it up for a friend's business. But I've talked to others who have worked with it, and apparently they feel it stacks up with commercially available e-commerce kits quite well.
Of course, that might speak more about the competition than anything!