Random Text and Attributes Scroll Quickly; Periodic, Slowly

[Grady Martin]

Hello, all.  The following two pads have me baffled.  One comprises randomized 
text and attributes, structured:

https://nosuckdotorg.files.wordpress.com/2015/09/curses-mod-data-random-fast.png

The other comprises periodic text and attributes according to that same 
structure, repeated every 16 lines and 32 columns:

https://nosuckdotorg.files.wordpress.com/2015/09/curses-mod-data-periodic-slow.png

All scrolling is vertical, and, except for the pads themselves, all factors are 
equal.  Yet, the randomized pad scrolls quickly, while the periodic one scrolls 
noticeably slower.  Why?