Performance of SD cards can be pretty variable - the class of the card isn’t a reliable indicator of the speeds you’ll get.
You can find a good benchmarking script and results for a selection of cards here http://www.pidramble.com/wiki/benchmarks/microsd-cards (spoiler - Samsung Evo+ microSDs are a good bet for the common Pi workloads).
I had a go benchmarking a PNY Performance Smartphone Edition Class 10 16 GB card that I’d bought in a local Currys, and got the following results under Ubuntu MATE 16.04. The card seemed particularly sluggish in use so was intrigued what results it’d show…
hdparm buffered: 12.20 MB/sec
dd write: 5.3 MB/sec
4K rand read: 4.59 MB/sec
4K rand write: 0.23 MB/sec