Forgive me if I repeat anything said and acknowledged here already, but a few notes. I'm no expert, but, I have spent alittle time studying the subject.
I own a well crafted, 10th to 11th century viking sword repro that is 39 inches long, and has a blade of 32 3/4 inches. it weighs in at about 2 7/8 pounds. The balance point is about 7 inches up the blade from the guard. It can be a bear to handle, for any extended period, although I will admit, I'm not a massive guy. I am however somewhat athletic. I can do a seated military press of 175 pounds, and bench about 230-245. From the skeletal record, Scandinavian raiders of the time where on average about 5' 9", and I would imagine built like a hard worked farm hand with a leaner diet, coincidentally, this almost describes me to a tee. So my point to this description of myself, is that I am probably as large, and as strong, as your average Viking raider, which, according to the record,is significantly larger than the rest of the Western European population, for the most part - Thus I figure, I would assume, I'm probably a good model for what the average or slightly more than average western european soldier could handle, in terms of sword weights.
There is, no doubt, to researchers, that medieval/dark-age blades weight, has been grossly over-estimated. But there are several factors at work here.
First of all, if you notice, 12th century and later blades, change in form - they taper towards the point more, and because of this, are much easier to handle for extended periods. Some have brought up the fact that this change in form was due to, singularly, an advance in understanding, and a need for thrusting at weak points in superior armor to that of the pre 12th century.. This is not completely true.
One thing that often is overlooked, is the PRACTICAL reality, of what your local blacksmith had as materials, and forging quality.
If you look at the historic record, many swords in northern/Western Europe, were made of scrap, and/or several rods of uneven, and questionable quality, twisted together, because of a lack of materials. The twisting technique however remained long after, and adds great strength, however, in pre-12th century europe, it was almost a necessity, just to keep the less than ideal materials, with a less than ideal temper, from breaking.
Also, tempering techniques, whether the knowledge was there or not, was most of the times not ideal. what this means is, that swords where desinged often with less of a taper, and more actual metal because given material and temper, a taper would result in a sword breaking under stress.
If you will notice also, as we reach the 14th century, the taper and length of sword becomes more exagerated. This is, basically, because the improvement in temper, teqnique, and available refined materials, was, better, AND a tapered blade, practically, is a superior tool.
Along with improved temper and material, the blacksmith could as noted earlier, now produce a two handed sword, with a blade thinner and more tapered blade, than a one handed 11th century sword. Thus the 5-6 pound Greatsword that is found later at the end of the medieval period. The fact that some Claymore's weigh in at 8 pounds, is more than likely due to the fact that the scottish were using Forges, and materials, that the more advanced civs had abandoned by the end of the 11th century.
My point however, is that, a one hander weighing 2.5-3 pounds, is at the upper end of practicality, unless it has an unusual amount of balance towards the handle, which is near impossible without a taper, and a taper, was not practical until after the 12th century, given materials, and temper common to the times.
An 8 pound Claymore, is yes, very possible, but not a result of practicality, but more likely a result of pre 12th century materials, forge, and forging techniques, in a 14th century and later setting, or a practical understanding that the 'smashing power of such a weighted blade, was as important as speed - Or an adaption since a 5 pounder was much harder to produce.
And indeed many two-handed/great swords did weigh in at 5 or 6 pounds, and sometimes even less.
And let me tell you, until you feel these weights in your own hand, and swing the thing around for 20 minutes, its hard to understand, that these are very significant weights to deal with, even if they sound rather low.