PHP and MySQL are actually very fast, check out Tech Empower's benchmarks (https://www.techempower.com/benchmarks/#section=data-r10&hw=...).
Remember Facebook is built on PHP. For this purpose (distributed servers) it's a perfect stack. Easy to code for and very fast. Security is one of our primary concerns and I assure you that Thndrstrm has our users' privacy and security in mind at all times.