Slow access to Percona Server 5.7 with current libraries

We recently upgraded our core servers from Percona Server for MySQL 5.6 to 5.7, running on Ubuntu 16.04. Right afterwards we experienced huge delays on our front-facing servers (also Ubuntu 16.04).

One of the query where we noted issues:

-- `users` is INTEGER NO NULL
-- `deleted_at` is DATETIME
set @rand_id=floor(rand() * 1000000); explain SELECT `users`.* FROM `users` WHERE `users`.`deleted_at` IS NULL AND `users`.`id` = @rand_id limit 1\G
Query OK, 0 rows affected (0.00 sec)

*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: users
partitions: p4
type: const
possible_keys: PRIMARY,index_users_on_deleted_at
key_len: 4
ref: const
rows: 1
filtered: 100.00
Extra: NULL
1 row in set, 1 warning (0.44 sec)

1k , which took the time for 1k queries down to ~2.5 seconds.

Can anyone explain why we’re seeing much better query times by switching from the client library for Percona Server 5.7 to that for 5.5 ?

Client App is Rails 3.2 (yes, I know; no, we can’t upgrade it ATM) using ActiveRecord, running as Passenger inside nginx.