trick down 90% and then swap to another pet ot let focus go back up seems to be the best way to do it, but yes, it takes time. Typically if you log out and come back in all pets will be back up to 100% after so many logout minutes. Not sure on what the timer is but I think it's 15-20 minutes to move back up from around 70% to 100... but yes, depends on the pet. common pets are easier to train than 'hard' pets. Also Mindark keeps fiddling with tt values on nutrio, so some pets may have used more in the past than they do now so it's possible if you shop around to find some that actually have more nutrio than the max. Support tickets on those cases to see if it affects xp are just copy/paste, we'll forward it on to another department stuff with no real answers. At least one avatar around here believes taming pets up increases loot potential due to decay costs, but I don't think it quite works that way.
vs rings pet buffs can cost more depending on the pet and rind in question...
My suggestion is to just feed the critter and trick down to 90 focus when not doing other stuff. After all pets are down to 90 log out and come back at it in a few hours. If you do it this way you'll be logging out sooner than you might otherwise, which can make peds last a bit longer than normal sometimes.
Also, I'd stuggest making sure buff you want is on the pet you are taming. Single buffers just don't seem to have a lot of market value currently. If system changes in new game engine that may change. Double buffers are barely worth skilling up, but there is a market there, at least somewhat of one. If a pet costs more than 1k I'd be careful about that since selling it off later you may not get back what you pay.... however, in some cases it can pay off if you find the right buyer. (sold ultra rare event pets for quite a bit more than I paid a while ago... boxer for about 3k more than I paid, and oratan axe for about .5k more than I paid.... you don't get sales like that often.... Had those pets about 2 years skilling them up before the sales finally happened a few months apart... )