Why is row lookup much faster than column lookup

[A J <s5...@gmail.com>]

>From my tests, I am seeing that a CF that has less than 100 columns
but millions of rows has a much lower latency to read a column in a
row than a CF that has only a few thousands of rows but wide rows with
each having 20K columns.

Example:
cf1 has 6 Million rows and each row has about 100 columns.
t1 = time.time()
cf1.get(1234,column_count=1)
t2 = time.time() - t1
print int(t2*1000)
takes 3 ms

cf2 has 5K rows and each row has about 18K columns.
t1 = time.time()
cf2.get(1234,column_count=1)
t2 = time.time() - t1
print int(t2*1000)
takes 82ms

Anything in general on the Cassandra architecture that causes row
lookup to be much faster than column lookup ?

Thanks.