I'm having the same issue with my networked Brother MFC 440CN.
B&W scanning is very fast with Simple Scan, but color takes forever (even at 300dpi setting). It seems that Simple Scan is setting the scanner to scan at the highest quality regardless of the DPI setting for color, and it's causing the scanner to move the scan head back and forth as it scans along.
I'm having the same issue with my networked Brother MFC 440CN.
B&W scanning is very fast with Simple Scan, but color takes forever (even at 300dpi setting). It seems that Simple Scan is setting the scanner to scan at the highest quality regardless of the DPI setting for color, and it's causing the scanner to move the scan head back and forth as it scans along.
XSane doesn't have this problem.