Tiki rely on MySQL, PHP and include Unified Indexing (I will add ElasticSearch).
Dozens of system and website use similar technologies to publish millions of pages that are consulted by hundreds of people simultaneously.
I experienced Tiki being used to publish several thousands of products from trackers using a little less than 100 fields + complex filtering + search system +++. Huge and worked ok.
You have to monitor and look your stats. Check what can be accelerate from content weight to queries (avoid overloading pages, simplify queries, cache, set proper boundaries to visitor path/search) on a weekly basis. And, yes, optimise tiki code for the good of your specific usage and all the community.
Once you master this part you can start by adding proxies, using CDN, load balancing, etc... working on your infrastructure. Google, there are plenty of good discussion about this.
Some people are lazy and they do the contrary.
It doesn’t solve issues, it just make them go away faster.
Keep us informed of your progress Anupam and good luck !