Hi, so I think we all know the classic problem of combat being too slow and boring but I have the exact opposite problem, my players go through my enemies so fast and there's no genuine challenge anymore. We play online so my other fear is if combat is too slow they get bored and lose focus (i swear players are like herding cats sometimes, love them tho)

I have talked to them if they are ok with combat being a bit more of a challenge and they're cool with it, but it feels like even if I up the AC and HP of enemies they still slash through them like soft butter. I am also trying to have enemies be more strategic with how they fight but I think because we play online (thanks rona) its harder for that to come across.

Either way anyone got some tips or ideas on how to slow down combat just a bit but not so much that players get bored?

Focus less on HP/AC and more on actions/damage. A quick fight can still be thrilling as long as it feels dangerous.

First, make sure your side is making just as many attacks as the party - factor in Extra Attacks, AoEs, pets and companions, etc. Action economy is huge in this game and if you're making 4 attacks per round while the party is making 16 you are not going to be a threat no matter what else you're doing.

Scaling up damage is also a great way to get players' attention. When one hit from the giant's club takes half the monk's HP, everyone suddenly sits up with their game faces on.

Messing with this too much can make for a quick party wipe though. I'd ratchet it up gradually until you have a good idea what the party can truly handle, and give priority to more actions rather than more damage because the latter can make things very swingy. Too many actions can be swingy too, but you can use them to do things other than attack in the event that PCs are going down and you want to avoid a death spiral without being too obvious in your mercy - maybe they try to drag away the downed PC or do some kind of ritual victory dance for their blood god.

If you're balancing encounters using CR, skew encounters toward the Deadly end more often than not. CR assumes a 6-8 encounter day to factor difficulty, and most groups just don't get that many. 

Quote from CharlesThePlant >>

If you're balancing encounters using CR, skew encounters toward the Deadly end more often than not.

If you're using the rules in XGTE, a decent rule of thumb is to just design for twice the party size.

Combat gets boring when all you do is whittle down a baddie's HP. It has nothing to do with speed or platform - it has to do with repetition. If you introduce obstacles, changing situations, and strategy, a fight that lasts one round can be the most harrowing event of your life.

The tensest, most nerve-wracking combats I've ever been in were online. And they were white-knuckle rides precisely because of the stakes ("Our long-dead bard was turned into a vampire! We have to beat the BBEG to save him!"), the terrain ("We're on flying mounts and there's lava beneath us! If our mounts get KO'd or we fall off, we'll die!"), and the baddies' intelligence ("The enemies are blocking our way out and taking potshots from cover! How can we draw them out without getting massacred?"). Think less about the monsters and more the scenarios you are throwing at your players. A fresh party can nova baddies easily because action economy is almost always on the players' side. But if they're outnumbered, if they are pinned down, if they can't see through magical darkness, if they can't communicate with each other, if the ground under their feet is crumbling away...that's when things get interesting.

One of the most enjoyable fights I have ever had was in a level 12 one-shot. I was playing a monk/barbarian, so literally all I could do was punch. It would have been boring as all get out, except the monster we were fighting was a dragon that changed colors when it hit certain damage thresholds. Yeah, we were just whacking away at a single monster the whole time, but it was engaging because we had no idea what breath weapon was coming at us next. The unpredictability made what could have been an HP slog an extremely memorable fight.

Tl;dr - Remember that you're making combat encounters. The where and why of a fight can matter just as much, if not more, than what is attacking. Broaden your scope and don't be afraid to get creative! :-)

You could increase monsters HP, this has for effect to drag combat. Also use of terrain, distance, cover, obscurement and other limiting factor can also affect the combat. Using monsters with vulnerability or immuninty to character's damage is also a way to make it last longer.

...cast slow on your combatants.  :D 

I think a great way to slow down combat is to employ something I call "cinematic combat." This is a homebrew rule that I began from the outset of my campaign. Essentially, the rule is that every player describes how they land the killing blow on every enemy they kill. Many DM's will narrate the death of the NPC's as the party moves through them, usually letting a player describe his/her kill on the final monster, or on a Natural 20. Why though? Let your players describe in detail how exactly their PC finishes off each creature they drop to 0 HP. I do this for several reasons:

1. This allows players to feel far more engaged in the fight as they get to weigh in on exactly what their PC has done 

2. It allows the DM not only a small break in narration during combat but also lets the DM sit and enjoy the creativity of the players

3. Its called "cinematic combat" because the entire group gets to see the varying fighting styles of each PC since its comes directly from them, with their own unqiue twists.

4. This is the most relevant point for this topic - it slows down combat significantly because the players get overly excited about each time they land a killing blow and they take the time to narrate their heroics, letting the evocative language of a combat scene marinate. Even more so, I always have the other enemies react to how they witness their comrades being killed. When the half-frost giant Rune Knight PC in my campaign tells me that he "brings his maul hand-over-hand crashing down on the Deurgar's skill, splattering bone splinters and brain matter across the dungeon floor as blood and gore soak his icy beard (yes, this actually happened), I can simply return with "As you turn to regard Duergar allies off to the side, they recoil at your once-blue face now stained red with their brethern's lifeblood. They grip their hammers tighter, give a great huff that somewhere between a war grunt and an exasperated sigh, and dig in their boots." 

Point being, this rule adds a greater depth of narration that also includes the players and combat will go slower and I think be far more enjoyable - without even having to touch HP or AC!

Good luck!

Have you read The Monsters Know What They Are Doing? It has great tips for monster tactics that can make fights really challenging and engaging. 

Could always use the video game tactic of making lesser bosses random encounters through the game. Have them fight multiple at once and whatnot, increase their level, hp and whatnot.

I think increasing HP and maybe having some mechanics in the fight that the will add the boss or big baddy are always a good go to. This way your players feel the challenge by they are not getting rocked by a mob that is just hitting hard because its over scaled. There are no rules, you as the DM can decide when the fight is over and how long it lasts. :)

You could always throw a monster at the players like The Line. Combat could last for a while because The Line has multiple options to retreat and many traps to whittle down player health and make the final showdown more high stakes.