I'm assuming the issue is that RasPlex is cataloging while it creates the menu? Yet this is in reference to a 4 GB card, which seems like a pretty small source to catalog. If it's just a GUI interface that is generally slow, welcome to the pi; it has the horsepower of a really low end smart phone. I.e. this very likely has nothing to do with the card. However...
Before you do anything, you should at least benchmark your current card using this test, keeping in mind that if the card is slowing things down in this context, it's because of the read speed.
On that chart of benchmarks, there are UHS-1 cards from Patroit, Sandisk, and Sony. Notice they all have about the same read speed, ~20 Mb/s. Also notice that this is the same speed as most of the class 10 cards, and, in fact, that no one has reported read speeds significantly faster than that for anything. If you look just above that test, there's another chart about what cards work on the pi, and some of those are UHS-1 cards with casual reports of read speeds (beware, the numbers in that chart are not speeds -- but look in the notes). Again, nothing above ~20 MB/s.
When I got the pi I tested two cards, a class 4 and an class 10, and while there was an appreciable difference in write speed,1 the read speed is identical (~20 MB/s). If you have a look at this thread, some one explains this in terms of the limitations of the card reader itself, which makes sense.
So if you get the 20 MB/s read already, buying a better card won't improve on that. I just tried the same benchmark with a USB (spinning disk) hard drive, and got 31.9 MB/s write and 27.5 MB/s read. Thus you could try moving the system files to an external drive, which might speed some things up a bit.2 Keep in mind though, the most frequently used chunks of system get cached in RAM anyway. Also beware that you'll likely need a powered USB hub to run an external drive (or a drive with it's own power supply).
1 Subsequently I tried a Sandisk class 10, which has twice the read speed of the Adata, but still the exact same read.
2 I don't recommend this because, TBH, I think it's pointless for this purpose. It's also a tad complicated.