That's a huge question and isn't as simple as to many elements.
I had a major FPS drop at one point, but it was due to having 3 cams active on screen at once. ( My cure was to add an on/off option for the 2 extra cams, so if your system is up to it, you can use them, or turn them off).
PS this crappy desktop i'm using now has 800Mb or so of ram, and my graphics card has 256Mb.
My desktop system sucks, but my laptops graphics card, went for shit, and the laptop needs 2 or 3 reboots till the graphics card clicks and works.
( my laptop, an HP Pavillion ZD8430ca XP MCE is like 7 years old now, i'm surprized it even boots up still & works..
.
I'm expecting at some point the laptop graphic card will just up & die.. but till then i use it daily for extra duties, like HD storage, twitter, FB ect..
I use the desktop for 3drad and 3dsmax, photoshop etc... and keep installed utility's to a minimum to improve performance.
To little ram surely wont help windows or any program work at it's peak, not just 3Drad.
Processor speed can be a limiting factor too.
In 3drad it's self, if your running elements ( turning them on/off for no valid reason, for instance, that can waste CPU cycles).
if you using scripts, ( i've heard using multiple scrips , and maybe connecting them to each other is a bad design, apparently it's best to do as much as possible in each script and don't connect them if possible ( using Val1 to Val9 for instance)).
There are limits to scripts too, i'm not sure of what they are exactly off the top of my head, but something like 119 element links per script max, if you have more then that the script may not work properly and may not even show an error, it just wont work right and you'll just keep trying to fix it, until you realize, there's nothing wrong with your script, it's something else..
)
As you can see, it's an involved process, and basically, you need a pretty good system & graphics card, and design your programs well too, to get the best performance out of any program you run, not just 3drad.
Intermix any of the things i've mentioned, and who knows where your problem may be, and probably 0 chances of anyone fixing your issues by just saying do this or that and you're good to go...
You have to look at a few things and combinations that mite be the problem.
And lastly, 3drad isn't perfect, it has some annoying little issues, that piss you off from time to time..
but.. it's free, & abandon-ware now, so what do you expect.
WOW did i type all that, i've had to much caffeine again.
, I've made a game that I will make "hard, fun, compete", I've made a lot of scripts, timing, and so perfectly according to my imagination, but I can not play it because fps is always at 0, there is a suggestion to add my fps while many elements?