Zombie Villager spawn rates are way below 5%, which used to be true in previous versions and is stated on wiki. Got 1 Zombie Villager in about 250 Zombies playing survival and just running around killing mobs. Which corresponds to 0.4%.

I know this is not the biggest sample size, but the numbers are way off what they should be. If you can, please do more large scale testing to find exact numbers.

Due to this, curing villagers gets really grindy, if not impossible, having to spawn 500 Zombies before getting 2 Zombie Villagers in survival. Considering Zombies are 1 of 4 mobs that spawn often, that makes 2000 monsters to spawn for 2 zombie villagers.

Edit: After doing some research I figured the spawn rate went down in 1.12. In previous versions the spawn rate was about 10% of all zombies being zombie villagers.