Jhin's reload time cannot be reduced, if I remember correctly, and it's quite long.
During which, he'll either be running away or trying to stay relevant with his Q grenades or similar.
Harass him as much as you can without endangering yourself too much.
His fourth shots do execute style damage, so stay out of his range if you have low health at more or less any possible cost, unless it's worth dying.
He'll either spend it on something else, or it'll automatically reload (within 11 seconds if I remember correctly).
His Q does more damage as it kills more per bounce, and it can chunk for quite a bit if it hits a champ on one of the later or last bounces, so be careful with wandering around minion waves. The maximum distance it can jump is quite short, so try to stay away from minions that are a prime target for the bounce.
His W is probably his second skill, but unless the support is one that actually does some attacking and such, he'll generally be limited to his own attacks to trigger the mark. His Q grenades do not make the mark, nor does a W. His E, his autos, and ally champ autos will make the mark. However, it's limited to the first champ it hits, even with a long range, and outside of point blank (basically within immediate camera range/just a short bit zoom out when he has his support or otherwise safe), he'll probably be farther away and pan out to get long distance roots off. Which means, he'll be a lot more vulnerable during such times. Also, it gives a precursory line to where it'll strike, so avoid it if you can.
His E are his traps. They give a slight range of vision around them, and they can be stacked, but subsequent ones will deal less damage. It's quite easy to usually step out of it even if you do trigger one, then the only predicament is to avoid his W that will probably come afterwards. Use some common sense and it'll be relatively easy to guess. He'll most likely place it between bushes, short entrance ways, along perhaps the middle of the lane if you're prone to going ahead of your minions, and so on and so forth. Let minions roam ahead if you can in lane to try and avoid such scenarios. The damage they can do can hurt quite a bit if it successfully explodes on you (aka, probably if he rooted you), so try to be careful with those. Something he can do though is stack the traps for repeated slows to try and catch you in at least one of them between the slows and a potential W.
A thing with its passive. If he gets a kill off, it'll create another flowering trap where their body was, meaning it can do quite a bit of repeated damage in teamfights between his damage, his executes, and his roots. It probably feels tough to do so, but if a teammate with low health is caught out near Jhin, he is most likely doomed, and unless you KNOW you can save him, it's better to cut free and try and at least stay alive, so they didn't just die in vain so you died too.
His ult. He'll probably be back quite a safe distance in most cases, as he's entirely and absolutely vulnerable during using it. He cannot spam all four shots, there's a short delay between each. For each shot (out of the four) that he doesn't use, part of the CD is refunded. They apply a significant slow if they hit, meaning it'll be easier to hit subsequent shots. The shots do more damage the lower health you have, so if you have low health, try and stay behind teammates with higher health if there are any, if you can't escape quickly (and I mean, QUICKLY). The last shot will crit for a huge amount of damage, and it can shred people apart. For the love of god, do not run straight back unless you are basically outside of it anyways. Run to the side, but realize he'll probably figure that out as well, and try to avoid it or hide somewhere with width, not length (aka, a bush going across the width, not straight backwards). Unless he has vision, he won't see anything besides the area just around the bullets he fires. So if you're in the Fog, you're a bit safer, but don't get complacent.
He's decent at poke in laning phase, but is hampered by his reload time a lot. His W is quite thin, so try to dodge it when you can. He has almost no mobility, and basically no peel outside of rooting someone which will probably not work out that well in a desperate chase situation (for instance, in a gank). So take advantage of that and see if you can get the jungler to harass him if he extends too far at all. He hyperscales into a monster in late game once he gets his items, because the amount of damage he can do is just incredible, and his W is amazing in utility, not to mention being able to substitute it for a makeshift ult in some cases.
IDK, a few cents.
