Importing diff files into a Nominatim server is very slow

I have a server where I installed Nominatim, everything works fine but daily imports are very slow. Here is the command I use to import updates:

./utils/update.php --import-osmosis-all --no-npi

and here is part of the log from postgresql (it seems it takes 20 seconds to import a single point):

2018-09-28 09:59:26.723 UTCLOG: duration: 20841.622 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 09:59:26.723 UTCDETAIL: parameters: $1 = '1991172' 2018-09-28 09:59:47.644 UTCLOG: duration: 20919.765 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 09:59:47.644 UTCDETAIL: parameters: $1 = '1990890' 2018-09-28 10:00:08.440 UTCLOG: duration: 20794.188 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:00:08.440 UTCDETAIL: parameters: $1 = '1991076' 2018-09-28 10:00:29.293 UTCLOG: duration: 20851.333 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:00:29.293 UTCDETAIL: parameters: $1 = '1991075' 2018-09-28 10:00:50.135 UTCLOG: duration: 20841.083 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:00:50.135 UTCDETAIL: parameters: $1 = '1393309' 2018-09-28 10:01:11.061 UTCLOG: duration: 20924.034 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:01:11.061 UTCDETAIL: parameters: $1 = '559848' 2018-09-28 10:01:31.961 UTCLOG: duration: 20899.036 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:01:31.961 UTCDETAIL: parameters: $1 = '464022' 2018-09-28 10:01:52.865 UTCLOG: duration: 20902.475 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:01:52.865 UTCDETAIL: parameters: $1 = '382641' 2018-09-28 10:02:13.821 UTCLOG: duration: 20953.867 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:02:13.821 UTCDETAIL: parameters: $1 = '131642' 2018-09-28 10:02:35.042 UTCLOG: duration: 21219.991 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:02:35.042 UTCDETAIL: parameters: $1 = '2019829' 2018-09-28 10:02:55.909 UTCLOG: duration: 20864.235 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:02:55.909 UTCDETAIL: parameters: $1 = '2019724' 2018-09-28 10:03:16.708 UTCLOG: duration: 20797.321 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:03:16.708 UTCDETAIL: parameters: $1 = '2004606' 2018-09-28 10:03:37.567 UTCLOG: duration: 20856.796 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:03:37.567 UTCDETAIL: parameters: $1 = '2004610' 2018-09-28 10:03:58.393 UTCLOG: duration: 20823.782 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:03:58.393 UTCDETAIL: parameters: $1 = '945985' 2018-09-28 10:04:19.219 UTCLOG: duration: 20824.386 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:04:19.219 UTCDETAIL: parameters: $1 = '1087915' 2018-09-28 10:04:40.215 UTCLOG: duration: 20994.079 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:04:40.215 UTCDETAIL: parameters: $1 = '525557' 2018-09-28 10:05:00.994 UTCLOG: duration: 20777.332 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:05:00.994 UTCDETAIL: parameters: $1 = '432880' 2018-09-28 10:05:21.794 UTCLOG: duration: 20798.616 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:05:21.794 UTCDETAIL: parameters: $1 = '1440909' 2018-09-28 10:05:42.733 UTCLOG: duration: 20936.692 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:05:42.733 UTCDETAIL: parameters: $1 = '530458' 2018-09-28 10:06:05.307 UTCLOG: duration: 22572.536 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:06:05.307 UTCDETAIL: parameters: $1 = '1487369' 2018-09-28 10:06:26.257 UTCLOG: duration: 20947.926 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:06:26.257 UTCDETAIL: parameters: $1 = '471227' 2018-09-28 10:06:47.082 UTCLOG: duration: 20822.594 ms execute index_placex: update placex set indexed_status = 0 where place_id = $1 2018-09-28 10:06:47.082 UTCDETAIL: parameters: $1 = '1983016'

I have another server with the same postgresql.config file which works without this problem. Is there any way I can improve the performance?