Slow jobs on Sidekiq

In my application I have a job which makes several database queries (mysql) to recalculate certain fields of a model. (A kind of counter_caches).

Launching individually these methods take about 10 seconds to finish, but when done by sidekiq take about 180 seconds.

The current configuration is a concurrence of 20 and the configuration of the pool in the database.yml is also 20.

I tried adding another instance of sidekiq, and the times are kept around 180 sec, but of course, twice as many jobs are executed simultaneously.

Is there anything I'm not considering? Is it normal for this to happen?