The game command /fps displays the "frames per second" and a bit of additional information atop the screen. Another code, /framerate, can be used instead. It shows the below information, and a bit more.
Diablo II runs at 25 frames per second. If your machine can display more than that, from 26 up to 125 or infinity, you'll see the game running at full speed. If your machine can't display that many, you'll see some stuttering and jumping of objects on the screen. More explanation below.
- FPS: How many frames per second the game is displaying at on your machine. (This number goes up and down depending on your machine's power, how many objects are on the screen, other players, etc.) The fastest possible display in Diablo II is 25.
- Phys: Physical memory on your machine. How much is being used/free.
- Ping: How many milliseconds packets require to travel from your machine to the server, and back. Determined largely by your internet connection. Faster is better for game performance. Over 300ms becomes laggy. Over 1000ms (1 second) is unplayable, for most character types.
- Display: Whether you're viewing Diablo II in 3d, and in what type of 3d mode. Requires a 3d card. (Which virtually every machine has now, but not everyone did back in 2000, when the game was released.)
Diablo's Internal Speed
Diablo II runs at 25 frames per second. The /fps will only display 25 (or less) on single player. On multiplayer it will display however fast your machine can handle. Values above 25 have no affect on game performance, since the game only updates every 1/25 seconds. A machine that displays at 150 FPS will not differ in any way from one that displays at 25 FPS. The difference comes when a slower machine drops below 25 when there are a lot of objects to track on the screen. Visually-busy effects, such as flames, eat up a lot of computer power and drop the frame rate appreciably.
There is one large difference between single player and multiplayer.
When the action grows too fast for your machine to maintain 25 frames per second, things seem to move in slow motion, since your machine takes more than 1 second to show 25 frames. The speed is locked in single player, so all (or most) of the 25 frames will be shown. Eventually. (This is much less of a problem now than it was back in 2000 when the game was first released, and computers were much less powerful.)
Multiplayer Diablo II runs at 25 FPS, regardless of how well your machine can keep up. If your machine's FPS falls below 25, the game continues running at the normal 25FPS (since it must keep in synch with other machines it might be playing with) and this results in choppy, slide-show style action. The lower your FPS drops, the choppier the display.
A slow machine with a lot of action to track might drop to 5 FPS, which means the display would only refresh 5 times a second, rather than 25. This means for every frame you saw, 4 would pass unseen. Since monsters, projectiles, etc, continue moving at 25 FPS, they will seem to jump from place to place, just like cars moving down the street when your vision is periodically blocked. Or like dancers under a strobe light.
Another key aspect of Diablo II's display is the fact that it's a 2D, sprite-based game. Every bit of action seen on the screen is an actual frame of animation. Since various movements (such as swinging a weapon, casting a spell, running a step, etc) are drawn in advance and displayed with X number of frames, faster attacks can make the movement look choppy, since frames are skipped.
For instance, if a normal axe swing takes 1 second, there are 25 frames of animation provided to display it. With faster swing speed from equipment, that speed might drop to .8 seconds, which would require 20 frames. The game would therefore drop 5 frames from the animation; it's not possible for the game to squeeze 25 frames into .8 seconds, since you only see the screen refresh 25x a second, at most. If you added more fast swing, from spells and equipment, and the speed dropped to .6 seconds, that would mean the animation only uses 15 frames. The same swing shown with 15 frames instead of 25 frames would be noticeably jerky or choppy, and if the attack got even faster, that problem would be exacerbated. This is why some very fast attacks in Diablo II look kind of sketchy, and it's simply a limitation of the technology available at the time the game was created.