Jessica had some good strength feats, but she is still a mediocre fighter. She could potentially one-shot with a clean hit (she was able to knock Danny out cold with one punch when they had to restrain him), but she had trouble handling more skilled opponents. At least Luke now has showings tagging IF, Murakami, disarming Elektra, and seemingly besting Sowande offscreen, but Jess had a lot of trouble tagging the Hand leaders. Though, to be fair to Jess, the Fingers of the Hand are all extremely skilled fighters, with several lifetimes worth of combat experience. I wouldn't be surprised if they actually outclass Batroc in terms of pure skill.
Jessica isn't actually that slow. If anything, Defenders showed that she has decent natural agility and reflexes. There were numerous instances where she comfortably ducked, dodged and avoided blows from various opponents, including multiple sword slashes from Elektra. And she was plenty capable of tagging lower tier Hand members, who all have a degree of training.
She isn't even that bad of a fighter. She's a decent brawler. But her skill level is still what lets her down against high tier opponents. Because the thing that is often the case with her, against highly skilled fighters, is she gets countered. She's often able to avoid initial blows aimed at her, but the moment she attacks, she often gets countered in some way, and then tossed around or floored, because her attacks and technique are sloppy compared to her opponents'. In some cases, she can still power through (like the Hand guy who was doing surveillance in the one episode), but she couldn't really do that against the main bad guys.
But, as I pointed out before, the Fingers of the Hand are not your average bunch. Gao could Tk push things as heavy as cars around. Sowande knew pressure point techniques that were able to cause Luke more pain than automatic gunfire does (even if the strikes didn't actually do any lasting damage). Murakami took multiple super strong hits throughout the show and kept coming back, was able to stagger Luke with some hits (when IF's normal attacks, a speeding SUV, numerous thugs etc. couldn't budge him at all), and somehow managed to disappear mid-air during his one encounter with Daredevil and Jessica. Hell, even Bakuto, who was the weakest of the Fingers IMO, completely ignored multiple gunshots to the chest, during the season finale.