First, the uncompressed index has a size of 3017824 K, and the times of two consecutive scans are
real 2m35.547s user 2m18.172s sys 0m6.146s real 2m36.304s user 2m17.152s sys 0m6.242s
Next, the compressed index has a size of 1164228 K, and the times of two consecutive scans are
real 3m20.342s user 3m4.939s sys 0m8.805s real 3m20.269s user 3m3.866s sys 0m8.699s
Since the system time went up I wonder if some extra buffering would work, also since this machine has two cores I wonder if I could do the scan in parallel.
Uncompressed with buffering makes it slower:
real 2m42.024s user 2m15.048s sys 0m6.214s
Compressed with buffering is slower too:
real 3m34.062s user 3m3.281s sys 0m5.488s
I ran each timing multiple times, and the times were mostly the same for each run.
So far straight reading of uncompressed data is fastest when scanning the whole data set.
At a later time I will try and make a parallel version.