Okay so I really like the Sims (3!) But .... [consultation/ventilation post]

#2 - Could be caused by damaged or corrupted custom content, outdated mods, or if your computer
isn't strong enough to handle so much content.Try removing your most recently added mods or CC and
see if that fixes the load time.Never had issues with store content giving me issues though.

As far as I know, it isn't possible to only install certain items from a world. It's all or nothing. You haven't mentioned your system specs either.

This is a comprehensive write up - https://www.nraas.net/community/TIPS-FOR-BETTER-GAME-PERFORMANCE

Be discerning with your custom content. Don't simply download just for the sake of it. Ask yourself whether you really need it, or if something from the game or an EP might suffice. I try to keep my CC folder at 3 gigs or under, because of performance. Sometimes broken or badly made CC can cause slowdown in your game. Try to stick to creators that are known for making good quality CC, and always read the comments or reviews before downloading to see if anyone has experienced any issues with the content. Usually, if there are three or more people who have all experienced the same problem, I either download warily and test for myself, or skip it.
TS3 will reward you tenfold with wonderful gameplay if you take care of it, but it does require a lot of kid glove handling to make it run nicely.

This mod here - http://modthesims.info/d/478307/remove-stencils-overlays-from-build-buy-cas-objects-in-game-edited-01-16-2014.html has been a godsend in my game. It has reduced my need for custom content simply by being able to remove stupid stencils or ugly overlays ( and speculars ) that EA have put on some game items that would be nice if not for ugly additions. No more cloth jackets that look like shiny leather, or pretty shirts with horrid stencils on them.

Check to make sure that you don't accidentally have any Sims 2 packages mixed up with your Sims 3 packages if you play TS2. Those will slow your game down immensely due to the games inability to read them. You can also follow the steps here - http://simswiki.info/wiki.php%3Ftitle%3DGame_Help:Package_File_Lag Using Process Monitor can help identify problem files. I still check this way sometimes, and it helped me find some blinds, which were a bit of a problem in my game. Once I took them out, it was good.

Clear your caches , etc, the usual. I also delete my world caches after every second or so play session. Not everyone does this, but its part of my TS3 voodoo ritual :D My game runs nicely so I keep up with it.
Other than that, I've never played the game using Origin, and would never, so I'm afraid I can't help you with the Fast Lane thing, except that I've seen people say the game / Origin unticks EP's or SP's by itself and that you should probably check that horrible Origin isn't deciding for you which SP's you should be playing with.

Someone more qualified than me can probably determine if your hardware is a bit underpowered or something, if you post the specs.

I can here to second what @Pary said. That link in particular to the tips for better game performance really helped me get my game to where it ran smoothly and consistently.

to add on what Pary said, I also like to convert any CC .simpacks to .package files for easy removal.
http://modthesims.info/d/364038/delphy-s-sims-3-pack-multi-extracter-updated-5th-sept-2009.html

@Pary
Thanks for providing the simwiki link! giving it try, though its a bit confusing lol.
Edit: I just used it and for some reason nothing shows up for my mods folder, even though I got stuff in it.
Edit: Got it to work.Every thing seems okay in my game except for one particular file, but its a library file?

@Avatarit - Trying to troubleshoot without having seen a DxDiag report is very difficult or impossible, so it would be good if you could run one on your computer, copy it all, upload it to Pastebin and put a link to it in your next post here.

How to gather information on your PC using DxDiag.

Thank you all for your comments!!!
A few notes:
1. Regarding the computer specs, I am thinking of maybe upgrading some of the parts, but it may take me some time (and in any case I will post in the technical forum). I was looking for solutions to implement as of now.
2. I have reinstalled all (/most) of my store content in the last few days,I want to update that I actually did managed to install specific items from store worlds- it is a little manual but it goes like this: I installed the world, then, in launcher - installed content- I deleted the world , it important to delete the package of the world first (I think i manually deleted from Installed worlds folder, too). Then I manually deleted from launcher all of the unwanted items from this world. Since I reinstalled all content and the worlds were pretty much at the begining it was easier, it is more difficult to locate items from world wheb there are many items installed. And so for example from Midnight Hollow I kept like a few dresses, a few hairstyles and the Cashier objects. I did this with 2-3 more store worlds.
3. I finished installing all store cc and other cc (using pre saved .dbc files), then ran one of my heavy saves. It actually worked pretty good!!! Imprtant note is that I ran this save with almost no mods.
4. Which bring me to the next question: I wish to convert most of my package files into sims3pack (I know most simmers actually prefer the opposite). I found only one tool to do that, which was not successful, and I can do this also with TSRtool but some items will not open there.