I think you should factor in the ability to adopt new tools and techniques...for instance Americans or Scandinavians are good capernters coz they have power tools...you'll find many nairobi carpenters compared to say mombasa or TZ or uganda...have adopted more tools and techniques...that make their work easier. The same with masonry...it nearly standard now to find cement mixers and little masonry tools in any construction site in Nairobi...this is what makes labour more productive and cheaper in the long run.
But key predictor for better labour productivity I dare say is the level of education (and of course capital..loans and savings come into play). It is easier for someone who has gone to form 4 and some college..to buy a cheap chinese power machine or laser cutter...than std 3 drop out at the coast..who apprenticed into a job.It is also easier for them to check the internet, download the latest designs and howtos...
Of course the slothfulness (due to culture) of TZ and coastal people need to be addressed. Where folks engage in fitina in job or just do not see the need to hurry up.
How about the average labour cost?
I have heard about the "Lazy" label. I don't buy it at all. You should compare a masonry or carpentry job done by a Mpwani to one by Mumbara as see for yourself. You can get highly specialized fundis at the coast able to turn a sandy, muddy / mangrove wall in to something harder than concrete. What is more is the workmanship. Whatever is made looks neat and not half baked. No corners cut.
I can concede that Kenyans may be hampered by corner cutting. You give them cement to use and only half ends up on the building. No wonder buildings are collapsing daily.