Summary of this thread from my viewpoint:
1) EE is plenty capable of running big sites with lots of traffic.
2) EE sites that require 100+ custom fields and 10’s of relationships *may* have performance issues. Performance is most likely to suffer if many custom fields and relationships have to be pulled onto one template.
3) EE performance issues associated with #2 can probably be addressed by writing “raw” queries (with or without Query Module) and having intelligently managed and reasonably powerful servers.
In my case, I don’t have any choice but to pull ~100 custom fields (some with relationships) into a single-entry template and ~10 custom fields (some with relationships) into multi-entry templates. Thus, if I run my whole proposed site with EE, I clearly need to consider that some raw queries are likely.
BTW, I still haven’t had time to investigate the alternative of integrating EE with a custom DB - more specifically a custom set of interrelated mysql tables in the same DB as EE. I’d be grateful if anyone wants to shorten my research and share key threads or experience with integrating EE and non-EE tables.
My thanks to all who’ve participated in this thread (so far). The EE community is pretty amazing!