2500 subscriptions per minutes on a shitty $5 digital ocean droplet (in 2013). Don't put the cart before the horse, 99.99% of applications don't reach even 100 concurrent users on a regular basis.
I used to shun SaaS web apps due to expense - "Layer messaging, $25/month, I'll build my own. When I get thousands of users it'll be too expensive."
Then I thought - it'll take me more than 6 months to build something which doesn't even scratch the feature set they have. Better use the tools and services available to build rapidly now, get to market and swap it out later.