I'm assuming this is for Plants vs. Zombies 1 on Android mobile because my game suddenly runs extremely slow as well. It happened after my game updated to the April 12th update.
"Your zombie-zapping experience will get better with some behind-the-scenes updates and brain-picking improvements."
Not sure what exactly they changed based on this, but it seems to have broken the game for Android phones as it can barely run once you start playing the game. Only the main menu areas and almanac run smoothly. Hope this gets fixed soon as I paid to get rid of ads and have been enjoying this game since 2009 on PC and since I've had it on my phone.