One Pest can't make a big trouble alone. You can have 6 of them in a deck, which means 6 quanta drain per turn. If you multiply them or their effect (eg. Fractal), they can drain a lot of quanta indeed, but there are way more powerful combos in the game... Besides, Sanctuary is not the only way to counter it - as it was mentioned, a single Quantum Pillar generates 3 quanta per turn. Supernova gives 22 (it costs 2 to play, so 24-2), etc. (Even the Pest-Spammer Decay can be defeated easily if you manage to play a few Supernovae in the early game.)
Pest is not overpowered.