Original ASIX Sigma software is using (faster) asynchronous FTDI transfers. Unfortunately libftdi in asynchronous mode doesn't return with timeout when it receives smaller than required data frame. It simply hangs in ftdi_transfer_data_done(): http://developer.intra2net.com/mailarchive/html/libftdi/2016/msg00118.html We should probably switch from libftdi to libusb: http://sigrok.org/bugzilla/show_bug.cgi?id=878
Since the async operation reportedly cannot be used with the external fdti library, I'm closing this item. The current driver implementation keeps returning to application code, which increases UI responsiveness. Speedup of the sample memory download could get addressed later, most probably after eliminating the ftdi lib dependency, ideally in common usb support code of the libsigrok component.