Restoring a table is ten times slower on Ubuntu 14.04 than on Ubuntu16.04

testdb=# \d photos_searchlog

Table "public.photos_searchlog"

  Column  |           Type           | Collation | Nullable |                   Default                    

----------+--------------------------+-----------+----------+----------------------------------------------

 id       | integer                  |           | not null | nextval('photos_searchlog_id_seq'::regclass)

 created  | timestamp with time zone |           | not null | 

 updated  | timestamp with time zone |           | not null | 

 lang     | character varying(2)     |           | not null | 

 q        | character varying(255)   |           | not null | 

 hits     | integer                  |           | not null | 

 count    | integer                  |           | not null | 

 ip_list  | text                     |           | not null | 

 locked   | boolean                  |           | not null | 

 ts_list  | text                     |           | not null | 

 ts_count | integer                  |           | not null | 

Indexes:

    "photos_searchlog_pkey" PRIMARY KEY, btree (id)

    "photos_searchlog_lang_q_key" UNIQUE CONSTRAINT, btree (lang, q)

    "photos_searchlog_count" btree (count)

    "photos_searchlog_created" btree (created)

    "photos_searchlog_ts_count" btree (ts_count)

    "photos_searchlog_updated" btree (updated)

It's only the statement ALTER TABLE ONLY photos_searchlog ADD CONSTRAINT photos_searchlog_lang_q_key UNIQUE (lang, q); which causes the delay. I use the default postgres configuration on the same hardware (/etc/postgresql/10/main/postgresql.conf). I tested different postgres versions, checked the locale and other settings but can not find any differences. I also tried with more or less data, but always the same result.