The average wait time on the phone to speak to a firm is now about six and a half minutes, compared to just over four minutes a year ago.
The energy price cap, which limits how much suppliers can raise the cost of household bills, went up by 700 a year to 1,971 in April for the typical household. Bills are expected to increase by 800 a year in October.
Wholesale gas prices have gone up in the past few months, which has resulted in a rise in bills. As economies were reopened after Covid restrictions, demand increased, but Russia's invasion of Ukraine has caused fears of supplies, pushing prices up further.
The increase in the wholesale cost of gas - the price at which energy companies buy it - caught many suppliers out and made them lose money on price promises to customers. Many of the smaller firms went bust and millions of customers were shifted to new suppliers because of this.
The charity said that it was particularly worried about people on prepayment meters who are at risk of having no gas or electricity if they can't afford to top up.
At a time when customers need all the support they can get, it's worrying to see service performance is the worst on record, said Dame Clare Moriarty, chief executive of Citizens Advice.
Citizens Advice said companies were getting slightly slower to respond to emails. In the same period in 2021, suppliers responded to 62% of emails within two working days, compared to 66% in the same period in 2021.
As these stats from Citizens Advice show that there are areas where customers are not getting the service they need and deserve in these very difficult times, Ofgem, the UK's energy regulator, said it was working with suppliers to improve service.