I have some forensics experience.
Material under the fingernails is removed with a sharp, sterile instrument that is essentially a pointed instrument that scrapes all loose tissue out. The nail is also clipped to catch any material that would be clining to the undersurface of the nail. You just can't get this material off by wiping with alcohol and it would take considerable time to do it completely on all 10 nails.
It can take weeks if you have to exclude multiple suspects. It gets very complicated if there are possibly multiple DNA profiles present on the same victim, of whatever the source (blood, saliva, semen, fingernail scrapings, oral/vaginal/rectal swabs).
The toxicology is also extended testing as anything found has to be accurately identified.
However, usually a pattern is starting to develop by two weeks of intensive work and the comment about narrowing investigation from 51 to 5 people makes some sense. 51 samples of DNA is a huge workload and I'm sure it's consuming massive amounts of LE Lab time and money.