Benchmark your card: Modules tab -> Bench.mo. After restart go to Debug tab -> Benchmarks -> Card Benchmarks -> Quick R/W benchmark (1 min)
I expect something around 70 MByte/s in write mode according to various benchmark tests for this card.
Make sure to adjust resolution and bandwidth for RAW/MLV recording according to results and don't forget not to run on theedge = There is some overhead involved. You are not allowed to expect the number seen in benchmark in RL recordings.
And using ExFAT file system may help, too.