Adding new field to big table

Hello,

I'm having time issues when adding new fields to a big table. I hope you can point me some hints to speed up the
updatesof a table with 124 million rows... 

This is what I do:

First I create a tmp_table with the data that will be added to the big table:

\d+ svm_confidence_id_tmp
                        Table "public.svm_confidence_id_tmp"
    Column     |       Type       | Modifiers | Storage | Stats target | Description
---------------+------------------+-----------+---------+--------------+-------------
 id            | integer          | not null  | plain   |              |
 svmconfidence | double precision |           | plain   |              |
Indexes:
    "svm_confidence_id_tmp_pkey" PRIMARY KEY, btree (id)



The table where this data will be added has the svmConfidence field

\d+ document;
                                      Table "public.document"
      Column      |              Type              | Modifiers | Storage  | Stats target | Description
------------------+--------------------------------+-----------+----------+--------------+-------------
 id               | integer                        | not null  | plain    |              |
 kind             | character varying(255)         | not null  | extended |              |
 uid              | character varying(255)         | not null  | extended |              |
 sentenceId       | character varying(255)         | not null  | extended |              |
 text             | text                           | not null  | extended |              |
 hepval           | double precision               |           | plain    |              |
 created          | timestamp(0) without time zone | not null  | plain    |              |
 updated          | timestamp(0) without time zone |           | plain    |              |
 cardval          | double precision               |           | plain    |              |
 nephval          | double precision               |           | plain    |              |
 phosval          | double precision               |           | plain    |              |
 patternCount     | double precision               |           | plain    |              |
 ruleScore        | double precision               |           | plain    |              |
 hepTermNormScore | double precision               |           | plain    |              |
 hepTermVarScore  | double precision               |           | plain    |              |
 svm              | double precision               |           | plain    |              |
 svmConfidence    | double precision               |           | plain    |              |
Indexes:
    "DocumentOLD_pkey" PRIMARY KEY, btree (id)
    "document_cardval_index" btree (cardval)
    "document_heptermnorm_index" btree ("hepTermNormScore" DESC NULLS LAST)
    "document_heptermvar_index" btree ("hepTermVarScore" DESC NULLS LAST)
    "document_hepval_index" btree (hepval DESC NULLS LAST)
    "document_kind_index" btree (kind)
    "document_nephval_index" btree (nephval DESC NULLS LAST)
    "document_patterncount_index" btree ("patternCount" DESC NULLS LAST)
    "document_phosval_index" btree (phosval DESC NULLS LAST)
    "document_rulescore_index" btree ("ruleScore" DESC NULLS LAST)
    "document_sentenceid_index" btree ("sentenceId")
    "document_svm_index" btree (svm)
    "document_uid_index" btree (uid)

Then I update the svmConfidence field of the document table like this:

 update document as d set "svmConfidence" = st.svmconfidence from svm_confidence_id_tmp as st where st.id = d.id;

But it takes too much time.
Is there something to take into account? Any hints?
Should I do it in a different way?

Thanks for your time...

Andrés
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