Which camera or device are you using the CF card on?
The requirements vary quite a fair bit in terms of read/write speeds.
I have a Transcend 64gb 400x CF card selling for $300 if you are keen....works perfectly fine for ProRES quality capture, but not fast enough for ProRES HQ capture on my Ki Pro mini.
It will work fine if you're using with DV-based CF card modules (ie. HDV/DV datarate), and also works fine if you're using 50mbps Canon XF camera, or even Nanoflash at lower bit rates.