The slowest grab in the game are Samus and ZSS's frame 16 grab and dash grab and their frame 17 pivot grab. Guess what? ZSS works it fine since she has setups and mobility. A frame 12 tether grab for her would be pushing it, but it's not insane and as people agreed on, her tether grab is what holds her from being a monster because of her throw game.
I don't know what Samus can do to confirm a grab, but like ZSS, her throw game is strong. Her lower movement speed does hurt her more than ZSS.
The fastest tether grab is Olimar's... I think it's Olimar's at frame 10 followed by the Links' frame 12 grabs. Seems long, huh? Let's look at the slowest melee grabs at frame 10, Zelda's, dash grab at frame 11, Zelda and Ganondorf's, and pivot grab at frame 12, Captain Falcon's. Following second slowest grabs are Bowser and Little Mac's at frame 9 and third slowest at frame 8, Charizard, DK, Duck Hunt, Falco, and Wario who doesn't care since all of his grabs are frame 8.
A frame 6 tether grab would be insane... or not? Or implies that if melee grabs on average were 2 frames shorter, then it might not be, but we'd be dealing with frame 4 grabs then for characters like Fox, Luigi, Diddy, and Sheik. Unchanged, it would be insane when you have melee grabs at frame 10 for Zelda and frames 9 and 8 for 8 other characters. That's 9 characters with melee grabs already screwed going to get screwed even more. What's more, 4 of them aren't heavyweights and who feel combos much harder than the heavies. Zelda, Little Mac, Duck Hunt, and Falco aren't having much fun with their slow grabs, especially Falco and Zelda who are slow moving characters. Little Mac is fast, but a frame 6 tether grab would murder him when he's out-spaced and read during approaches. At least Duck Hunt can throw stuff and stay out of tether grab range.
Would it help for Link and other tether grabbers to have faster grabs? Absolutely, especially the ones who don't get much off of grab like Yoshi or ones who can't really find ways to get grabs safely without being punished heavily like Pac-Man or Bowser Jr. At the same time, the slow melee grabbers should have reduced startup on their grabs too. There is no reason Bowser should have a frame 9 grab. Compared to DK and Charizard, he gets little off of grabs. Sure, he has kill throws, but so do DK and Charizard. Little Mac's ground game is supposed to be good and while boxers aren't known for grappling and he's wearing boxing gloves, Little Mac having a frame 9 grab isn't great. I don't think he gets much out of grabs, but his pummel is good. Frame 7 at the least. Zelda... Her grab doesn't even have range like Pac-Man's or Bowser Jr.'s and she's slower than both. Was Zelda purposely designed to be bad in general? Oh, she's good in doubles. So are Sheik and Villager.