Whether this is noticeable or not is questionable, but lens are where all the money goes.
That doesn't surprise me, as the frames are pretty simple to make, but the lenses have so many variables (including cutting to fit into the frames), and any mistake at any point requires the lens to be dumped and for them to start again from scratch, with the final QC being done on arrival at the opticians from what I can tell.
My father has complex lenses* and no matter what optician he's gone to there has always been a chance that they'd reject the glasses on arrival, IIRC one pair was apparently redone about 3 times before the optician was happy to hand them over, and he's had delays at least 4 or 5 times that I can remember where the optician has rejected other pairs (this has happened at Boots, Specsavers and the local indie over the years).
One of the reasons I tend to be "happy" to pay the price in the opticians is I know that if there is a problem they'll fix it, often within minutes of me popping in, as over the years I've had them replace lost screws, restring glasses, new pads etc all done free even on older pairs.
*For years he had to have 3 sets of glasses (long, reading, computer), with prisms in one lens, usually with a new set at least once a year:/ He now has to have one lens frosted
(apparently one of the opticians he went to was oddly excited to do that, he was a younger one and it was the first time he'd had the opportunity to frost a lens).