i have a 4GB class 6 and have no problems

 

I have tried even a sandisk 16gb micro sdhc class 10 UHS-1(Ultra High Speed Transfer) extreme pro 90mbs transfer speed didn't get the 90mbs but 25mbs isn't to bad

 

I'm using a 32gb Sandisk Ultra Class 10. It averages about ~20-30mbs, so our phone must have a limit of Class 2. So theoretically any card would have about the same performance on our phone.

On a side note: When I boot the phone up with my 32gb card in, the phone goes all kinds of screwy with the system processes force closing left and right. If I insert the card after the phone has booted up, it works perfectly. I guess the phone 'supports' 32gb cards, just not perfectly. I wonder if a custom ROM would be able to fix this.

 

The default sd cache size on the Hydro is (as far as I remember) 128 KByte.

If you increase the cache the speeds will get a bit better.

You can increase it either manually or download a free or paid app.

I am lame and use an app (SD-Booster), because the size limit is embedded in the kernel and will resize (default / aka shrink down to 128 Kbyte) after a reboot.

I have tried 1024 Kbyte cache and 2048 Kbyte cache, and for me 1024 works best, but you might also want to try what ever you feel like it.

Before you enable "set at boot" you should make sure that you're phone is stable. I heard if it is set too large the phone might become unstable, but I have never tried anything more the 2048 Kbyte anyway.

 

To quote myself:

I used to own the Design 4G, unfortunately I broke it and didn't have phone insurance, that's how I ended up with the Hydro ...

I did see a speed increase on the Design after increasing the SD cache size.

I have not performed (before/after) speed tests on the Hydro.