Currently I have the following but they're not good:
- Kingston 8GB Class 6: REALLY slow - tansfer rate below 1 Mbps
- Transcend 32GB Class 4: slow - transfer rate about 2-3 Mbps
Both were bought from Amazon and as you can see, even a Class 6 MicroSD can have lower performance than Class 4, depending on the manufacturer.
You can check your MicroSD card speed using this tool from Google Play. The screenshot from that app shows some insane card speed, as shown below: