Why isn't dump maximally efficient with TK70 tapes?

[Ian! D. Allen [CGL]]

DECsystem 5400, Ultrix 3.1C, RA90 disk, one user (me).

Watch the elapsed real times here.

Here's a plain root dump to tape (TK70):

    # time dump 0 /
      DUMP: Date of this level 0 dump: Thu Jun  7 21:17:43 1990
      DUMP: Date of last level 0 dump: the epoch
      DUMP: Dumping /dev/rra0a (/) to /dev/rmt0h
      DUMP: Mapping (Pass I) [regular files]
      DUMP: Mapping (Pass II) [directories]
      DUMP: Estimates based on 1200 feet of tape at a density of 10240 BPI...
      DUMP: This dump will occupy 1103 (10240 byte) blocks on 0.13 tape(s).
      DUMP: Dumping (Pass III) [directories]
      DUMP: Dumping (Pass IV) [regular files]
      DUMP: 57.43% done, finished in 0:03
      DUMP: 1103 tape blocks were dumped on 1 tape(s)
      DUMP: Tape rewinding
      DUMP: Dump is done
    0% real=9:29 usr=0.3 sys=1.9 rd=0 wr=4 mem=56 pg=3 rec=17 sw=0 sig=0 cs=2776

Here's the identical root dump piped to dd to tape:

    recorder# mt rew
    recorder# time sh -c "dump 0f - / | dd bs=32k rbuf=2 wbuf=2 of=/dev/rmt0h"
      DUMP: Date of this level 0 dump: Thu Jun  7 21:28:18 1990
      DUMP: Date of last level 0 dump: the epoch
      DUMP: Dumping /dev/rra0a (/) to standard output
      DUMP: Mapping (Pass I) [regular files]
      DUMP: Mapping (Pass II) [directories]
      DUMP: Estimated 11295744 bytes output to Standard Output
      DUMP: Dumping (Pass III) [directories]
      DUMP: Dumping (Pass IV) [regular files]
      DUMP: 11295744 bytes were dumped to Standard Output
      DUMP: Dump is done
    0+2780 records in
    0+2780 records out
    4% real=3:34 usr=0.7 sys=8.7 rd=1 wr=8 mem=37 pg=2 rec=17 sw=0 sig=0 cs=10111

That's almost three times faster!  Why can't dump be as good as dd?
Dumps are of major importance; I would have thought that dump would be
the most clever user of the tape drive.  I can't believe this.  Am I
missing something?  I must be missing something.
-- 
-IAN! (Ian! D. Allen) idallen at watcgl.uwaterloo.ca idallen at watcgl.waterloo.edu
 [129.97.128.64]  Computer Graphics Lab/University of Waterloo/Ontario/Canada