Epson ES-1000C Reference Manual page 155

The high-speed mode is suitable when scanning bi-level originals,
such as text and line art. When scanning continuous tone images, as
with 8-bit monochrome and 24-bit color scanning, use the normal
mode.
Note:
To obtain the full benefit of high-speed data transfer, use the high
speed scanning mode in combination with block-structure data
lock transfer.
The actual effectiveness of high speed scanning may be limited by
host computer conditions, such as processing speed.
Command Reference
3-61