The more difficult the synth, the more skill ups you will attain. This is easily noticeable and my best example of this in action was when I was doing my WW to 100. I was basically doing 10 levels of lumber to reach 100. Those materials clock in at just one log per synth. After 99 levels of woodworking I proceeded to watch as doing Staurobow synths, a vastly more difficult synth, got me from 99.2 to 100 in like 15 synths after I gave up on Jac lumber's poor poor performance. Easy synths are super easy but they come at a price.
The closer you get to a cap, the fewer skill ups you will achieve. It's almost never a good idea to cap on a synth due to slower skill ups, but sometimes it's simply for the best. Generally those times are only when you are looking at 50+, and it's really only Goldsmithing for under 60.
Having done alchemy to 93 before I gave up on a pointless run to 100(don't care for the 100+6 for anima after staves and WW, and my only reasoning when I started was easy crafts to 100 to aid my gameplay)I found that it blows for skill ups. Perhaps it's due to cheap costs because the synths overall are anything but easy imo, but skill ups in alchemy are a nightmare. I'm afraid you are stuck with those crummy skill up rates. Everything past 60 for me was just a damn nightmare, and it's worse than ever at 93(er maybe I'm 91 I forget)trying to cap on icar wings. Eventually, after throwing away about 15 stacks of giant bird feathers which is 30 synths total for like .2 skill up once upon a time on just ONE SKILL UP ROUND, I finally said @#%^ THIS and gave up on Alchemy. And I've never looked back.
Goldsmithing is also a good example of poor skill ups on easy synths. It's been a nightmare to skill that up. All of those 4x beastcoins and ores and nuggets and sheet synths are just plain slow and makes you want to tear your hair out because you do so many synths that any losses are just an after thought. You just want to progress already and losses be damned. It's only now on shrimp lantern synths that I'm seeing great skill up rates again, and those cap at 73. 58 to 68 took me months of preparing time and again for skill up runs because skill ups were so slow, and I refuse to power level and throw away millions of gil. I've bought ores and nuggets but that only makes sense and have even made money on ingot sales. The mob ingot stage was incredibly slow. It's really making me cringe about that 80-87 run on elemental beads just thinking about it. I'm fine with breaks and losing money but going over a dozen synths without a skill up is nothing new in Goldsmithing and just makes me want to drink bleach. Granted you will see it make up for itself when you get a few skill ups in a row but still, it's not always. I can say the same for quite a bit of my WW career during skill ups.
I guess what all of this boils down to is synths with sub crafts tend to skill up faster than synths with no sub crafts, much like a larger gap will skill up faster. In other words, more difficult synths make you learn faster, which is why you should rarely bother with capping synths. A good rule of thumb is to get 1-2 levels from cap and move on. It's almost always 1 but sometimes 2 is welcomed with open arms complete with a parade that would make the Star Wars universe proud. Cheap and easy synths are great to do, but not always the best option.