Hey buddy, I got a solution for you. I've placed some apps of mine to my sd-card, without lag or problems it was just working fine. Today in the morning I could not watch a video on my sd card, so i pulled it out and put it back in. After the restart some of mine apps were turned in to grey and nothing of mine data (picture, musics, apps) was avaible like yours. I went go to my pc and make back up of my SD card so I could Hard reset my phone. But before I did: See solution
BUG: SD card did not load the apps and media properly (it keeps saying: sd card is not ready and that i have to wait or reinsert the SD card when i press to an app which is inside of mine sd card).
SOLUTION: Get a SD Adapter or something like that where you can access the data on your micro-SD from pc. It will say that your SD card is broken and shows you a selection if you want to recover it. Go ahead and recover. It will succeed in minutes. I've done this and put my sd card back on my phone, and all my apps on SD-card did load up with all the data without any problems or slow down's like some people are issuing.
I am using Nokia lumia 520 and my sd card is a class 10(8GB).
Ive also noticed that a class 10 Sd-card makes really diffrence between slow downs.
Hope it will work for you guys that have the same problem to!
P.S: Sorry for my bad english, hope it is enough to understand