libusbK microframe OvlK_Wait

Question

I am implementing a driver for an USB isochronous device in a latency critical application. For my tests I am using the Cypress FX3 kit (USBIsoSourceSink example). The endpoint is setup to supply data at bInterval 1 (transfer every 125usec microframes ~= 8kHz).

My data packages are very small (32bytes/pck). I am allowing 8 packages pr transfer. Now, I am trying to demonstrate the 8kHz transfer rate in order to achieve a 125usec transfer latency. For the IN endpoint my approach is this: I have a vector of 4 transfer objects, each with a KOVL_HANDLE handle and a KISO_CONTEXT* context:

struct Xfer {
    UCHAR buffer[MAX_PACKETS_PR_XFER * PACKET_SIZE_IN];
    KOVL_HANDLE overlapHandle;
    KISO_CONTEXT* isoCtx;

    Xfer(KOVL_POOL_HANDLE &poolHandle) {
        IsoK_Init(&isoCtx, MAX_PACKETS_PR_XFER, 0); // StartFrame 0 = Asap
        IsoK_SetPackets(isoCtx, PACKET_SIZE_IN);
        OvlK_Acquire(&overlapHandle, poolHandle);
    }
};

I submit all 4 transfers by OvlK_ReUse(xfer.overlapHandle) and

UsbK_IsoReadPipe( usbHandle, pipeID_IN, xfer.buffer, sizeof(xfer.buffer), (LPOVERLAPPED)xfer.overlapHandle, xfer.isoCtx);

While the program is running I point to a transfer in the cue and OvlK_Wait(xferINiter->overlapHandle, 100, KOVL_WAIT_FLAG_RELEASE_ON_TIMEOUT, &transferred);.

Then I process (save) the data from the transfer and resubmit it (OvlK_ReUse and UsbK_IsoReadPipe) before I point to the next transfer in the cue, wrapping around.

This all seems to work. My problem is that I process 1000 transfers per second while I am expecting 8k. It seems that I am only getting the 1ms frames and not the microframes. I can easily get more packages per transfer, but my application is latency critical.

What am I doing wrong? Does libusbK have a limitation for only handling 1ms frames?