I tested with gunicorn as WSGI server.
Yes, I still have such delay. But for me, it was the fact the client is fetching the view. I have it only on first access (for each session).
For example, still with gunicorn, on a new session client, I have 3 sec delay (1 records in party):
[2020-05-28 15:11:31 +0000] [30664] [DEBUG] POST /test/
30664 8327690698608 [2020-05-28 15:11:31,248] INFO trytond.protocols.dispatcher <class 'trytond.ir.action.ActionKeyword'>.get_keyword(*('tree_open', ['ir.ui.menu', 51], {'client': '51c7a555-b1e1-4489-811f-e1a34a6cc444', 'language': 'fr', 'language_direction': 'ltr', 'groups': [1, 2]}), **{}) from admin@127.0.0.1//test/
[2020-05-28 15:11:31 +0000] [13129] [DEBUG] POST /test/
13129 8327462641168 [2020-05-28 15:11:31,610] INFO trytond.protocols.dispatcher <class 'trytond.modules.party.party.Party'>.fields_view_get(*(116, 'tree', {'client': '51c7a555-b1e1-4489-811f-e1a34a6cc444', 'language': 'fr', 'language_direction': 'ltr', 'groups': [1, 2], 'view_tree_width': True}), **{}) from admin@127.0.0.1//test/
[2020-05-28 15:11:31 +0000] [30664] [DEBUG] POST /test/
30664 8327690698608 [2020-05-28 15:11:31,769] INFO trytond.protocols.dispatcher <class 'trytond.ir.ui.view.ViewTreeState'>.get(*('party.party', '[]', None, {'client': '51c7a555-b1e1-4489-811f-e1a34a6cc444', 'language': 'fr', 'language_direction': 'ltr', 'groups': [1, 2]}), **{}) from admin@127.0.0.1//test/
[2020-05-28 15:11:31 +0000] [6106] [DEBUG] POST /test/
6106 8330549218480 [2020-05-28 15:11:31,849] INFO trytond.protocols.dispatcher <class 'trytond.modules.party.party.Party'>.view_toolbar_get(*({'client': '51c7a555-b1e1-4489-811f-e1a34a6cc444', 'language': 'fr', 'language_direction': 'ltr', 'groups': [1, 2]},), **{}) from admin@127.0.0.1//test/
[2020-05-28 15:11:32 +0000] [13129] [DEBUG] POST /test/
13129 8327462641168 [2020-05-28 15:11:32,520] INFO trytond.protocols.dispatcher <class 'trytond.modules.party.party.Party'>.search(*([], 0, 1000, [['name', 'ASC'], ['id', None]], {'client': '51c7a555-b1e1-4489-811f-e1a34a6cc444', 'language': 'fr', 'language_direction': 'ltr', 'groups': [1, 2]}), **{}) from admin@127.0.0.1//test/
[2020-05-28 15:11:32 +0000] [13129] [DEBUG] POST /test/
13129 8327462641168 [2020-05-28 15:11:32,569] INFO trytond.protocols.dispatcher <class 'trytond.modules.party.party.Party'>.read(*([1], ['active', 'code', 'code_readonly', 'lang', 'name', 'replaced_by', 'tax_identifier', 'lang.rec_name', 'replaced_by.rec_name', 'tax_identifier.rec_name', 'rec_name', '_timestamp'], {'client': '51c7a555-b1e1-4489-811f-e1a34a6cc444', 'language': 'fr', 'language_direction': 'ltr', 'groups': [1, 2]}), **{}) from admin@127.0.0.1//test/
[2020-05-28 15:11:33 +0000] [6106] [DEBUG] POST /test/
6106 8330549218480 [2020-05-28 15:11:33,574] INFO trytond.protocols.dispatcher <class 'trytond.modules.party.party.Party'>.resources(*(1, {'client': '51c7a555-b1e1-4489-811f-e1a34a6cc444', 'language': 'fr', 'language_direction': 'ltr', 'groups': [1, 2]}), **{}) from admin@127.0.0.1//test/