Hi, just unwrapped two units of CRS326-24G-2S+RM, updated them to 6.39.2 and plugged a couple a 10G sfp+ to test it’s capabilities. I fired a btest between them and it seems that I can’t get more than 1Gbps. See the image attached.
It seems that there’s a cap somewhere to 1G, but don’t know if it’s hardware / software limitation, or that I just have to setup something differently..
Really, are you running Btest from the switch??
You need to run it from 1 x86 to other x86 through the switch.
Currently I don’t have any 10G capable device other than these two switches, and I specifically (maybe naively) bought those instead of the CSS326 so I could use winbox, and also btest, to check that the fiber link is working as expected.
CPUs on CRS series are there to run RouterOS and use some ROuterOS features not to generate 10G of traffic 
For 10G traffic generation you need full out CCR device and use Traffic Generator.
macgaiver is right, the device is not capable of GENERATING random data in such amounts. It should be tested for it’s switching speed, not random data generation capacity