General SD card's affect on speed

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http://androidforums.com – When I bought my phone I was intending to expand the memory with a 32gb SD card. However the salesman in the shop, who had previously shown very little knowledge on anything, advised me that using anything larger than 8gb would slow everything down and would grind to a halt with 32gb. I was intending to download Memory Maps, which is over 12gb but could hold the main file in my PC and just load specific files as required. However with the larger card I could hold everything on the phone as I'll also need room for MP3 and photos. Anyone had any problems with large SD cards slowing everything (General)