Slow update statement

[Reposted from General section with updated information]
Pg 7.4.5

I'm running an update statement on about 12 million records using the
following query:

Update table_A
set F1 = b.new_data
from table_B b
where b.keyfield = table_A.keyfield

both keyfields are indexed, all other keys in table_A were dropped, yet this job has been running over 15 hours. Is
this normal?

I stopped the process the first time after 3 hours of running due to excessive log rotation and reset the conf file to these settings:

wal_buffers = 64 # min 4, 8KB each

# - Checkpoints -

checkpoint_segments = 128 # in logfile segments, min 1, 16MB each
checkpoint_timeout = 1800 # range 30-3600, in seconds
#checkpoint_warning = 30 # 0 is off, in seconds
#commit_delay = 0 # range 0-100000, in microseconds
#commit_siblings = 5 # range 1-1000

Would it just be quicker to run a JOIN statement to a temp file and then reinsert?

TIA
Patrick