It's CPU-bound. It's single-threaded. It relies heavily on vector graphics, and they're composited in a very sub-optimal way.
Each graphical layer incurs a FPS cost. Every frame is "built up" by drawing the successive layers on top of each other, from background to foreground. So the game draws the guy's nude shin, then his nude thigh, then his nude torso, then the lower section of his trouser leg, then the upper section of his trouser leg, then his shirt (note: this is
not a complete list). IIRC there's a culling system for occulted GFX layers, but it doesn't deliver much improvement; human input is needed. You can speed up the game by removing unnecessary clothing (such as the male character's pants) and disabling optional details (such as his testicles). Your goal is to reduce the number of on-screen graphical layers which the game needs to render.
There are also non-obvious layers. Dynamic hairstyles in SDT are drawn in separate strands, with each strand being rendered as a "chain" of small sprites linked at pivot points. The hair might seem to consist of only a few sections (behind skull, in front of skull, in front of ear) but a complex hairstyle could actually include 10+ layers. As mentioned above, additional layers will hurt your FPS. The "physics simulation" workload needed to animate hair strands is negligible; this is all about poorly-optimized GFX.
The game includes many small visual details - such as saliva, mascara streaks, lipstick smears on the penis, and semen. Such details help to make the scene more real/immersive - but they also hinder the game's performance. The game can cull excess semen strands, but it's a slow/weak process (it doesn't want the user to
notice any sudden disappearances, so it allows the FPS to degrade quite badly before it begins to tidy up). The in-game menu has a button which you can use to manually clean the scene, so remember to use it when things become messy (and FPS degrades). There should also be a keyboard hotkey (via Mod Loader) to instantly trigger a cleanup.
Suggestions for improving FPS:
- reduce the Flash image quality (to the Medium setting, or even to Low if you're desperate)
- run the game in windowed mode (not fullscreen)
- manually shrink the game window (e.g. 800px X 600px) so that it fills only a corner of your monitor
- use static hairstyles (the ones which behave like "helmets") rather than dynamic hairstyles (the ones with moving/animated parts)
- disable optional SDT features which you don't want/need (such as lipstick smearing and saliva strands)
- remove excess clothing
- reduce the male character (there will be less graphical burden if he's modded down into a floating penis and a floating hand)
- avoid transparency. Never set an Alpha slider to 0%. Remove/unload/disable that element instead.
I encourage you to apply
ALL of these suggestions at first, just to establish a performance baseline. If you can obtain >40 FPS then you can safely experiment: enlarge the game window, re-enable some features, add more clothing/accessories, etc. Hopefully you'll discover a balance between fidelity and performance (at perhaps 25-30 FPS) which you can tolerate.
On the other hand: if you're still getting <20 FPS at minimum graphics, then you'll need to do something more drastic. You could download an alternate Flash Player, tinker with CPU core prioritization, or even look into CPU overclocking. But it might be more worthwhile to consider the various SDT successor projects. Some of them require a Patreon subscription, but they'll deliver
much better performance on your computer (because they don't rely on Flash).