The camera maxes out at about 20 MB/s on the SD slot, other cameras can be faster or slower.
The XX MB/s rating on most cards is the max read speed, write speeds are often quite slower. Sometimes a card that's rated to be slower will clear buffer because it has a faster write speed than the faster rated card, like the Lexar 600x and 800x CF cards as an example.
Write speeds are what's important to most photographers as they may want to clear the buffer as fast as possible, so important to see what a cards write speeds are and if you're using SD cards to know what your camera's max write speeds so you don't waste money(assuming you don't get another body that can take advantage of the faster card) on a faster that won't be any faster.