HM Inspectorate of Constabulary in Scotland (HMICS) said only four out of 14 deaths in custody since 2013 had so far been the subject of a published Fatal Accident Inquiry (FAI).
It recommended publishing regular data on deaths in custody, and said more could be done to speed up the FAI process.
HMICS said the figures in the report related only to deaths that occurred in a custody centre, not those which took place prior to arrival, such as that of Sheku Bayoh.
Mr Bayoh died in Kirkcaldy in May 2015 after being restrained by officers responding to a call about a man with a knife.
His family are awaiting a decision from the Crown Office on the case, with a FAI or public inquiry expected to be held in the absence of any prosecutions.
HMICS said: “Of the 14 deaths in police custody that have occurred since 2013, all of which were of men, the determinations of only four FAIs have so far been published.
“Two FAIs relating to deaths in 2013 are still to be held. While an FAI is pending, the Police Investigation and Review Commissioner’s (Pirc) investigation findings are not made public.
“The significant period of time that may pass before an FAI is held and a determination published limits the timeliness and potentially the relevance of any learning identified by the sheriff.
“Delays also have a profound effect on the family of the deceased, as well as the officers and staff who were involved in their care.”
The watchdog said “clearly more could be done to expedite” the FAI process.
Concerns have repeatedly been raised about the length of time taken to hold FAIs.
Figures obtained by the Lib Dems earlier this year showed some families had waited up years for an inquiry.
And a backlog of FAIs relating to the prison system means the families of prisoners who died as long ago as 2015 are still waiting for the outcome of an inquiry.
Lib Dem justice spokesman Liam McArthur MSP said: “This report simply reinforces what we already know about Fatal Accident Inquiries. The current process is plagued by systemic delays and a backlog of unanswered questions.
“Research by the Scottish Liberal Democrats showed that this is not just an issue for deaths in custody – families have waited up to ten years for the results of an inquiry. In the meantime, lessons cannot be learned and mistakes are doubtlessly being repeated.”
He added: “From any angle, FAIs are simply not doing the job they are meant to do. The system urgently needs overhauled.
“Despite recent additional investment, it may be that a new system of checks and balances, outside of the Crown Office, is required to provide the impetus needed.”