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.)
Plants not mentioned are unaffected.