Check this thread out, your looking for random read write speeds...
A lot of cards that are rated as class 10, only reach that speed when they are reading data from sequential blocks like a photo/video or music track for example.
And you can be dame sure this is the value they stick on the packaging lol.
They can be pretty rubbish when they need to read/write blocks from different parts of the disk like the Rpi needs it too.
So in fact some slower rated class 6 cards outperform class 10 cards when used for computing.
As a general rule there is little between SanDisk and Samsung cards, Samsung may have a slight edge I'd stay away from Kingston cards they are one of the worst offenders.
https://www.raspberrypi.org/forums/viewtopic.php?f=2&t=4076
You don't need to waste loads of cash on a super fast SD card either as the Rpi slot Max's out at around 22Mb I think !
Another thing you can do to help is reduce the resolution of your fanart and image resolution, and sometimes it's completely unrelated and its just that the site your scraping the fanart from is slow..