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Cookies are a confusing concept. I'll revisit what they are and how they work, and what, if any, speed impact there is in deleting cookies.

I keep reading that its a good thing to delete cookies to protect privacy. But if I delete cookies, won't it make my computer slower when it try to access those websites again?

Yes and no.

It won't slow down your computer in the way that you're probably thinking.

However, in general it will slow something else down.


Cookies: A Refresher

A cookie is a blob of data that is placed on your computer at the direction of certain web sites that you visit, and then provided back to that web site when you return.

For example: let's say you visit (not a real site). When your browser requests the page the web server sends back some data - something along the lines of "here's 50 bytes of data, and we'll name it 'mycookie'".

".. deleting cookies won't really make an appreciable difference in the speed of your web browser, your computer, or your internet experience. With one, huge, exception."

Your browser then, in addition to displaying whatever web page was returned, stores away three pieces of information:

  • The URL:

  • The name: mycookie

  • The data: in this case the 50 bytes of data that the website sent when you last visited.

That's a cookie.

Now, some time later - perhaps a few days later - you visit again. When you do so your browser connects to the web server and says (in effect): "I'd like your home page, and by the way, last time I was here you left this cookie - mycookie with these 50 bytes of data".

So a cookie is nothing more than data associated with a particular web site that is placed on your computer by that web site simply so that it can be returned to that web site the next time you visit.

Cookie Speed

Cookies are small.

Most cookies are significantly smaller than the web page you're viewing, and add almost no appreciable time to whatever it takes to download and display the page that caused the cookies to be downloaded.

In fact, even if a given cookie is already on your computer, there's a good chance that it'll be downloaded again anyway; often because it's easier, and often because the contents of the cookie have been updated by whatever you're doing with the web site.

So deleting cookies won't really make an appreciable difference in the speed of your web browser, your computer, or your internet experience.

With one, huge, exception.

Cookies and You

One of the many things that cookies are used for is to remember who you are.

I don't mean in any privacy invasive way, I mean quite literally who you are as you go from page to page on a web site. For example, I login, identify my user name and password, and the web site places a cookie on my machine that says, in essence "this is Leo, and he's authenticated". Now each subsequent page I visit on that site knows who I am.

In other words, I don't have to login for every ... single ... page.

Now, let's say in the middle of all that I erase all my cookies.

I have to login again to any web site that I was visiting that had stored my name or status in a cookie.

The page-to-page example is to drive home the point that cookies are often used in ways that are fundamental to how websites need to operate.

But they're also often used as a convenience.

For example, a website may automatically log me out after a while, but it can continue to remember who I am, in a cookie, so that days later when I return I don't have to re-type my user name, only my password.

Having to re-enter my user name would slow me down.

And that's where deleting cookies can slow things down. If you delete all cookies on your machine you will most likely find that you'll suddenly have to re-enter data that previously had been remembered for you in those cookies.

It's up to you. Me? I never delete cookies. I just don't see that huge a risk, and there's almost certainly no performance benefit to doing so.

You may feel differently.

But at least now you'll understand that while deleting cookies won't slow down your computer, it might slow you down.

Article C4500 - October 23, 2010 « »

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Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

Not what you needed?

Doug Brace
October 23, 2010 8:14 AM

I routinely delete my cookies, cache and temporary Internet files because when I close my browser, the next time I open that browser I want a clean slate.

October 23, 2010 10:01 AM

Perhaps you could explain why, when I sign in to Live Messenger, Microsoft finds it necessary to place THREE DOZEN cookies on my computer (one of which, if undeleted, will hang on till 2037)? I use Firefox and the CookieCuller extension and choose which cookies to retain, to avoid this sort of excessive tracking.

October 24, 2010 5:43 AM

I also like a clean slate each day and because Roboform fills and submits my username and password in one click, deleting cookies doesn't slow me down. For those cookies that are reqired to keep me from having to answer the pre-chosen questions, CCleaner allows me to choose not to delete them. It's a win-win situation: I get what I want and my financial sites get what they need.

October 26, 2010 12:49 PM

deleting cookies allows you to use "cashback" sites (a UK scheme - not sure if it's something you use in the states), so when you've browsed for you shopping item, you can return to that site via a cashback site and get the cash.

October 27, 2010 3:05 AM

But are they costing you money?
Recent reports in the UK & Eire showed that at least one budget airline [no names no extra baggage charges] was using cookies to adjust the price that you are being offered. Basically if you look at "flight to desirable destination arriving 10:00 a week next Friday" today it will store that info. If you do not purchase on that visit but rather close the browser and come back tomorrow and look at the same flight the it will increase the price [or not offer you the lowest which is the same thing]. Presumably on the basis that by coming back you have shown that you are already more likely to purchase and thus be less price sensitive.
Legal? - certainly
Ethical - debate
Costly - "You bet your bippy" [copyright Rowan & Martin or their scriptwiters or NBC or..]

October 27, 2010 6:36 AM

Would it be possible to copy cookies from one machine and use them on a different one? For example, could someone make a copy of the cookies on my PC and then copy them over to their machine, in turn gaining access to the websites I have visited and being able to log in with my credentials? Never thought of it before, but surely this type of thing isnt possible.

Yes. In practice, poorly designed web sites can be vulnerable to this. Fortunately it's also not that terribly hard to protect against - it's one of the reasons you may have to re-verify your password on a site you're already logged into, or one of the side effects of having inactivity timeouts.

September 16, 2011 6:55 PM

That's pure bullshit Leo. Cookies can overload your cache and slow down ones computer. And by the way, ever heard of tracking cookies that track your every step on the Internet and relay that information to whomever?

Bullshit? Thanks. My experience is exactly the opposite - cookies rarely adversely impact people's computing experience. And yes, in fact, I have heard of tracking cookies - I've written about them here: What are tracking cookies and should they concern me? - I'm guessing you won't like that article either.

March 9, 2012 2:55 PM

I recently found a way, within Firefox, to delete individual cookies. Tools -> Options -> Privacy; there is a link to remove individual cookies. I had HUNDREDS of cookies from sites I did not even remember. I spent a good hour going through the list and deleting anything that was not familiar. Now Firefox seems faster. But maybe it's only my imagination, running away with me.

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