Acho que você está ficando sem créditos da CPU na sua instância T estourável.
When an instance starts to run low on CPU Credits, its performance will gradually return to the baseline level (10% to 40% of a single core, as listed in the table above). This deceleration process takes place over the course of a 15 minute interval in order to provide a smooth and pleasant experience for your users.
daqui: link