Restoring a table is ten times slower on Ubuntu 14.04 than on Ubuntu 16.04

Hi,

I try to restore a table on U16.04, but it's ten times slower than on U14.04. This is the definition of the table:

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.

Does anybody have a clue what could cause the time difference?

Thanks