Lack of oversight and preparation for a European election count in Northern Ireland were partly to blame for long delays, a standards watchdog said.
The increased number of candidates and voters should have been more fully considered by the official in charge, the Electoral Commission added.
The count took a day longer than expected and DUP MP Nigel Dodds branded the process a travesty.
The Commission said: "There appeared to be a lack of overall management and oversight on the first day of the count resulting in some count staff not being used as effectively as they could have been."
Northern Ireland's chief electoral officer Graham Shields called for future elections to use an electronic counting system but said he could understand frustration after counting went into a second day.
There were 635,927 votes polled - an increase of 147,036 since the last European Parliamentary election in 2009. Ten candidates represented the largest number in recent European Parliamentary elections in Northern Ireland.
The Commission said: "The impact of an increased turnout and more candidates should have been more fully considered by chief electoral officer in the run up to the count."
Some count staff seemed to work considerably slower than others and delays were also caused by the long time taken to complete calculations at each stage of the count, most notably during the second stage when it took more than an hour to work out the transfer of approximately 4,000 votes.
On the second day, Mr Shields allocated an experienced official to oversee management of the count, which appeared to improve speed and efficiency, the report said.
"There was a lack of contingency planning in place with the general assumption that the count would be completed by mid to late afternoon based on previous European Parliamentary elections."
The chief electoral officer and his senior staff had to spend considerable time ensuring there were sufficient staff available to work the following day.
"As a result a number of staff who had no experience of working in counts were appointed to work on the second day alongside those who agreed to return."
The Commission acknowledged that by its nature a manual Standard Transferable Vote (STV) count will take time and that European counts in the Republic of Ireland and Malta also took a number of days to complete.
It added: "However, we think that improvements could have been made to the planning based on the likely turnout and number of candidates, and to the overall management of the count process.
"Such planning could have helped to better manage expectations on how long the count would have taken to complete, as well as to identify which parts of the count were running particularly slower than others.
"It is important that lessons are learned ahead of the UK Parliamentary General Election next May and the Assembly elections in 2016, where closely fought contests will bring increased scrutiny.
"The CEO's plans should identify the expected timing for completion of all stages of the count, and should be flexible enough to be revised to reflect changes if those assumptions change."
It said such plans should reflect every stage of the count including the verification of ballot papers, the sorting and mixing of ballot papers, the counting of votes and the adjudication of spoilt ballot papers as well as the process for calculations and transfers.
Mr Shields said: "The Electoral Commission were engaged in the planning for these elections and it would have been very helpful if they had raised the concerns set out in their report prior to the elections taking place.
"I am therefore looking forward to working with the Commission to examine what can be done to improve the efficiency of manual counting at STV elections in advance of the Northern Ireland Assembly elections in 2016."