Silex / Lumen
They both provide minimum bootstrap.
Reading your comment about millions of records - has nothing to do with your framework and has all to do with SQL queries and how you code.
People who bash Lumen/Laravel + Symfony/Silex because its PHP and hop into NodeJS because of raw performance doesn't understand the architecture of scalability.
I'm partial to Lumen because of pure community helpers - however if you try to do anything advance you might be stuck in a bind as i have faced. There are more out of the box packages compatible with Silex than Lumen/Laravel - however the community is getting more and more support to dev out compatiable drivers/packages. Watch out for documentation also - a lot of laravel base operations cannot be done in Lumen because it is a minimal framework - the minute you creep out of scope - you will want to ponder if it is not best to move to the big brother laravel in the end.