200 visits/second caused blank pages half the time.
300 visits/second? Boom.
Then again, 300 visits per second, estimating 250KB for the entire page (css, js, images, etc) is about 75Mbps which is going to be a lot to ask from any shared hosting provider.
What I do with my VPS:
1. WP Supercache is configured to write .html.gz copies to disk.
2. Nginx is configured to check if such a file exists before referring anything to PHP. If it finds the .html.gz, it will serve it directly from disk. Ordinary file caching gives it the advantages Varnish promises.
3. WP Supercache appears to correctly handle invalidation when a new comment is posted.
Wordpress can handle insane amount of traffic if you know what you are doing. 99% of wordpress borking under sudden spike in traffic issues can be boiled down to
- Not using caching
- Using shared hosting
It's all about the caching plugins. I had my blog hit 300k visits in the span of 8 hours, peaking at about 100 visits per second on a shared web host. Utilizing cashing plugins means every single person that visited my blog was served static content that refreshed itself every 10 minutes.