VACUUM ANALYZE taking a long time, %I/O and %CPU very low

While VACUUMing a large table, why aren't the CPU and/or I/O
percentages pegged?

I kicked off a VACUUM ANALYZE on a database containing a 20 million
row table (~250 bytes/row). It's been running for > 2 hours now, with
%CPU and %I/O rarely exceeding 1% (as reported by top), e.g.:

Tasks: 120 total, 1 running, 117 sleeping, 2 stopped, 0 zombie
Cpu0 : 0.0% us, 0.0% sy, 0.0% ni, 99.9% id, 0.0% wa, 0.1% hi,
0.0% si
Cpu1 : 0.3% us, 0.0% sy, 0.0% ni, 99.6% id, 0.1% wa, 0.0% hi,
0.0% si
Cpu2 : 0.0% us, 0.0% sy, 0.0% ni, 99.9% id, 0.0% wa, 0.1% hi,
0.0% si
Cpu3 : 0.3% us, 0.0% sy, 0.0% ni, 99.5% id, 0.1% wa, 0.0% hi,
0.0% si
Mem: 5977256k total, 5952096k used, 25160k free, 13604k
buffers
Swap: 4192924k total, 532k used, 4192392k free, 5827812k
cached

The most recent log entry was:
INFO: vacuuming "public.jimbo"

Output of strace (see below) does show blocks of 8K reads, so I
imagine the VACUUM is proceeding, but my real question is why isn't
one of CPU or I/O usage pegged to 100%?

select(0, NULL, NULL, NULL, {0, 70000}) = 0 (Timeout)
read(158, "\2\0\0\0\30!\32\307\1\0\0\0`\0h\1\0 \3 \230\236\312\2 "...,
8192) = 8192
read(158, "\2\0\0\0 B\32\307\1\0\0\0`\0h\1\0 \3 x\236\n\3\330\234"...,
8192) = 8192
read(158, "\2\0\0\0\10c\32\307\1\0\0\0`\0\210\1\0 \3 \230\236\312"...,
8192) = 8192
read(158, "\2\0\0\0(\204\32\307\1\0\0\0`\0P\1\0 \3 \210\236\352\2"...,
8192) = 8192
read(158, "\2\0\0\0\230\245\32\307\1\0\0\0d\0 \1\0 \3 @\236z
\3\200"..., 8192) = 8192
read(158, "\2\0\0\0\310\306\32\307\1\0\0\0`\0(at)\1\0 \3 @\236z
\3\200"..., 8192) = 8192
read(158, "\2\0\0\0\310\350\32\307\1\0\0\0d\0\220\0\0 \3 \230\236"...,
8192) = 8192
read(158, "\2\0\0\0\330\t\33\307\1\0\0\0`\0`\1\0 \3
\200\236\372\2"..., 8192) = 8192
read(158, "\2\0\0\0\20+\33\307\1\0\0\0`\0008\1\0 \3 @\236z
\3\310\234"..., 8192) = 8192
read(158, "\2\0\0\0\240K\33\307\1\0\0\0`\0\340\1\0 \3
\240\236\272"..., 8192) = 8192
read(158, "\2\0\0\0(at)l\33\307\1\0\0\0`\0\320\1\0 \3 @\236z
\3\330\234"..., 8192) = 8192
read(158, "\2\0\0\0H\215\33\307\1\0\0\0`\0h\1\0 \3 @\236z
\3\200\234"..., 8192) = 8192
read(158, "\2\0\0\0h\256\33\307\1\0\0\0`\0P\1\0 \3 @\236z
\3\300\234"..., 8192) = 8192
read(158, "\2\0\0\0\10\317\33\307\1\0\0\0`\0\320\1\0 \3 P\236Z
\3\220"..., 8192) = 8192
read(158, "\2\0\0\0X\360\33\307\1\0\0\0`\0 \1\0 \3 @\236z
\3\310\234"..., 8192) = 8192
read(158, "\2\0\0\0\340\20\34\307\1\0\0\0`\0\350\1\0 \3
\230\236\312"..., 8192) = 8192
read(158, "\2\0\0\0\3702\34\307\1\0\0\0d\0x\0\0 \3 x\236\n
\3\250\234"..., 8192) = 8192
read(158, "\2\0\0\0\0T\34\307\1\0\0\0`\0h\1\0 \3 @\236z\3\310\234"...,
8192) = 8192
read(158, "\2\0\0\0ht\34\307\1\0\0\0`\0\10\2\0 \3 `
\236:\3\270\234"..., 8192) = 8192
read(158, "\2\0\0\0\200\225\34\307\1\0\0\0`\0X\1\0 \3 @\236z
\3\200"..., 8192) = 8192
select(0, NULL, NULL, NULL, {0, 70000}) = 0 (Timeout)