This is what happens to plants slowed down by Frozen Peas and Frozen Watermelons:

Sunflower, Sun-shroom, Twin Sunflower, and Marigold: Sun and Coins are produced at half the speed.

Peashooter, Repeater, Threepeater, Split Pea, Snow Pea, Puff-shroom, Scaredy-shroom, Fume-shrooms, Sea-shrooms, Gatling Pea, Starfruit, Gloom-shroom, Cactus, Cabbage-pult, Kernel-pult, Melon-pult, and Winter Melon: They shoot projectiles slower.

Grave Buster: It will destroy the Grave slower.

Chomper: If chewing while slowed, it will take longer to chew.

Cob Cannon: If not reloaded when slowed, it will take longer to reload, but if it's already loaded, the cob will land longer.

Imitater: If it hasn't transformed into a Plant yet, takes longer to transform.

Umbrella Leaf: Has a short delay between deflecting.

Squash: Takes longer to jump into the air and squash zombies. (Credit to Lawn Defender.)

Potato Mine: If not armed yet, it will take longer to arm itself. Only Winter Melon zombies can do this. (Credit to Lawn Defender.)

Cherry Bomb, Jalapeno, Doom Shroom, Ice Shroom: Will take a longer time to explode.

Coffee Bean: Will take a longer time to awake the Mushrooms.

Blover: will take a longer time to blow away Balloon Zombies.