Same fish, exactly 13 months apart, in a 400L tank with a bunch of other plecs, fed Hikari wafters, NLS Mega and courgette.
There are several factors that could limit a royals growth, the two that spring to mind here are lack of food and stunting.
Yes Panaques eat wood, but it isn't and shouldn't be their staple diet. I feed mine Hikari wafters, NLS Mega Fish Formula, and veges like pumpkin and courgette. And they still find plenty of room for wood! I notice you have clown loaches, which could make getting food to a single little plec hard work. Make sure the clowns are well fed before lights out then leave some food behind for the royal. I've also had much better luck keeping multiple plecs as it allows you to be a bit more generous with the amount of food you feed, which makes it easier for them to find it (one plec + 1-2 algae wafers vs 10 plecs and a big handful of wafers), and I believe it also encourages a bit of healthy competition for food.
Between my friends and I we have noticed considerably different growth rates amongst plecs, some that have been sitting in the little 10g at the LFS for months and months (and who knows how long at the importer/wholesaler) just seem to never grow, where as fresh imports seem to be more vigorous.
Who ever claims one inch per year is nuts. Perhaps when they are older and get big, but it should certainly be more than that when younger. I estimate the one in the pics above grew from 6cm to ~20cm, so a 14cm or 5.5" increase over 13 months, and the other three royals in the tank had similar growth (which was considerably more than the 3 L14 in the same tank). And that was in a very crowded temporary tank, I'm expecting the good growth to continue now they have an 8x4' footprint to roam!
