I found that libusb calls are mainly responsible for long idle times.
 
For some improvement/workaround look at E-Mail with topic:  ftdi_write_chunksize, block_erasers and spi_read_status_register proposals
 
https://mail.coreboot.org/hyperkitty/list/flashrom@flashrom.org/thread/W2HULJTDPHWPBZY6MLM6TGT7RTHSGHON/
 
But still looking for further improvement.
 
Regards,
 
Simon