This question is probably bigger than you think.
First of all, using¬† echo base_url();?>¬†instead of "hard-coding" your web address will not slow down your site. A function call like this is¬†very¬†negligible to the speed of loading your pages.
I think the other part of your question is regarding¬†architecture.
When you think of¬†speed¬†for your website, you need to know what factors slow down the loading of your page. (Although not an exhaustive list, this will help in your case):
- the cache-ability of those files
- some server side header nonsense (e-tags and so-forth)
- the processing to build your php pages
- the size of your page
Now, in your instance, I would recommend putting all of your "static" files in the document root under a folder (say¬†static). Then, access them all in your "views" with the¬†base_url()¬†function.
This way, your page as it's delivered to the browser, will make external calls for those static files - allowing the browser to cache all of those files (assuming the headers are set up correctly). If you put them into views, then they're actually added to the page that is being requested. So, the next page that is requested has to download those files again along with that second page being requested. Make sense?