I do way more research for Urban Fantasy than I do for High Fantasy because in Urban Fantasy I'm borrowing from established mythology and folklore. For High Fantasy...What's that? What exactly is High Fantasy? I define it as a "second-world" setting; the story doesn't happen in an alternate version of a typically European country at some point in history. That sub-genre allows me great liberties and freedoms to build anew; however, I have to keep certain fundamental concepts and minutiae familiar so I don't lose the reader in irrelevant-to-the-plot details--like renaming an orange a pogwith; or that people walk on their feet, not their lips.
I learned a long time ago that there is such a thing as too much research; that it can cripple the story. Once upon a time, I fancied writing historical romance. WUT?? Stop laughing! My need for accuracy meant I spent more time researching than writing, and--in truth--not a lot of what I researched made it into the novel. It's the great "months of research ends in ten sentences." I'm not the author who can do historical accuracy well. There are those who can, and I'm an avid reader of their works. Bless them. For my stuff, Urban and High, I'm more of a "pinch of this, dash of that" researcher.
Here are my Top 3 Kinds of Research for Crafting Fantasy:
1. Minor Details
I'm not as smart as I want to be, so I have to look up what's probably common knowledge. For example, in Larcout, the first book of my Fire Born, Blood Blessed High Fantasy series, the type of an individual's innate magic is identified by the kind of rock in his/her forehead. I'm not a geologist or a gemologist, I had to look up "what stone is yellow and pretty?" Citrine fit the bill. Making up a name for it would've confused the reader.
2. Character & Place Names
Names are definitely a thing I research. Regardless of sub-genre. Sometimes I use totally made-up names like Vadrigyn or Beigreith. Sometimes, a made-up name has a plot-shifting meaning that will be revealed during the series. A lot of times, I take a "real" or "common" name and tweak it just a wee bit. Those more recognizable names serve to remind the reader of characters' general attributes. They also remind me of backstories (that only I need to know), including fatal flaws. Example: In my upcoming Urban Fantasy there are three Berserkers: Xipil, Hywl, and Runjit. Can you guess they're not pasty blonds from Scandinavia? Can you guess their origins? Yes, I'm riffing on the myth, and the names give you a hint.
3. Weapons & Armor
While my protags wield atypical weapons, and magic usually makes an appearance at some point, a lot of the supporting cast uses recognizable weapons and a few wear parts of traditional armor. Broadswords, flails, bevors, and vambraces. I research their proper names and jargon. I research the maintenance required for each piece. I research the material flaws, the situational disadvantages, and superstitions of each. This can be a rabbit hole if you veer into military tactics, battle history, weapons evolution. It's a fascinating rabbit hole, but... if you want to get in and get out, then limit what you're looking for.
So, there you go, three types of quick world-building research I do for my two types of fantasy novels. We won't discuss image-inspiration research. That's my weakness, my time-suck, my "just one more gallery and I'll stop (maybe, maybe a week later)."