i have a cheap Kingstone SD card and an Android phone with small internal space. when it told me to free some space i partitioned my SD into 2 partitions one fat32 for personal data and the other ext2 for extending my internal memory. using the app "Link2SD" i moved all the apps and games, but i noticed that apps performance had a dramatic change of speed (to the worse of course) Facebook app for instance it lags alot when i scroll, like, comment and move between tabs. for games they worked fine but they have the loading screen for a longer time. my phone is Sony Xperia M2 (if it matters). is there any SD card that has the same transfer speed as the phone's internal memory or even better. Thanks
You will likely not find an SD card which will outperform the flash in your phone, however there are substantial differences between memory cards.
Generally memory cards are labeled with a "Class" - which equates to the minimum speeds of the cards - Class 4 cards can handle 4 MB/Sec, Class 10 = 10 MB/s, UHS Speed 1 10 MB/s and USB Speed 3 30 MB/Sec.
Even within the classes though, there are huge difference in read and write speeds, and a good Class 6 card could outperform a bad class 10 card. There is a good section on speed ratings in the Secure_Digital Wikipedia article.