More than 1,800 cases are waiting to be processed by police, data collated by 5 Live Investigates has shown.
The most serious example, in Wiltshire, saw at least one case being delayed by more than 21 months.
Home Office minister Karen Bradley said the government had "prioritised child sexual abuse as a national threat".
Hi-tech crime units deal with the retrieval and examination of data from computers, mobile phones and other media devices, such as illegal images or details of inappropriate conversations with minors.
The evidence can be crucial in securing convictions against those who groom and abuse young people online.
More than half of the 40 UK police forces that responded to a Freedom of Information request from the programme reported delays of at least three months. Cases were "waiting to be allocated" to a member of staff in the hi-tech crime unit for analysis.
Five forces held devices that had not been examined after more than a year of the case being reported.
In Scotland, the oldest unallocated case dated back 10 months, in Wales, seven months, and in Northern Ireland, 18 months.
Louise began speaking online to a 35-year-old man when she was 13. He began to groom her and attempted to persuade her to visit him in person.
When her mother Amy alerted the police, she was astonished to be told it would take six months before forensic examination of her daughter's laptop and phone could begin.
"I was horrified, because what could they be doing in the six months?" she said.
The family eventually waited 10 months for police to examine the evidence, but the case was dropped after they said Louise's phone no longer worked.
"How they can't get the phone to work in today's day and age I don't know. God forbid [the abuser] has done something to anyone else," Amy added.
Both names have been changed to protect their identity.
In July, a report by Her Majesty's Inspectorate of Constabulary (HMIC) found delays of 12 months were "not uncommon", after examining 124 cases at random in six police forces.
It concluded such delays "increase the risk to children", with suspected offenders released on bail for prolonged periods of time while evidence was analysed.
Families of children who had been abused online said the failure by police to launch a full and immediate investigation into every case had left paedophiles free to continue their abuse.
Det Supt Guy Collins, of Lincolnshire Police, told the programme it was struggling to cope with the demand for hi-tech investigations into child sexual exploitation. It had a backlog of 10 months.
"We have to prioritise, that's the sad reality of life. We can't do everything straightaway," he explained.
He added that incidents involving high-risk suspects were always dealt with more swiftly than others, but admitted this meant other cases could be pushed back in the queue.
"It is a matter of absolute regret that we can't do all of those [cases] as quickly as we'd like to, but we do work very hard to protect children," he said.
Lincolnshire Police, like a number of forces, is now allocating additional resources to its hi-tech crime unit.
Karen Bradley, minister for preventing abuse and exploitation, said the government was "committed to ensuring police have the resources they need, and have prioritised child sexual abuse as a national threat".
She said the Child Abuse Image Database, launched in December 2014, "will reduce the time taken to identify illegal images".
5 live Investigates is broadcast on BBC Radio 5 live, Sunday 8 November at 11:00. Listen online or download the programme podcast.
Hi-tech police crime units dealing with incidents of suspected child abuse have developed a backlog of cases, the BBC has learned.