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The problem with slow response times is simple: Users are distracted in their interaction with the site. This makes it more annoying and difficult for users to buy products and pursue other goals.
Two main factors create slow download times: Large web page sizes and slow server performance.
Pages: Too Many Bytes
Web pages become large when they include code and content that wasn't optimized for the Web. For instance, pictures and graphics usually needs to be reduced in size to download fast. The same goes for pages built with too much code (long stylesheets included in each pages, many scripts, etc.).
Leading sites such as Yahoo strive to avoid large pages by dedicating developer resources exclusively to optimize page sizes and maintain fast response times.
Slow Servers
Page download will typically be slow when:
- many current users on the site. Once the server resources are gone, users will get an error message instead of the intended web pages. This is not the response you want when many customers visit your store.
- slow servers are in operation.
- servers turn to energy saving mode during night hours to save energy. This will cause problems to users browsing during night hours - and to international users in other time zones. The users will simply get an error message till the server wakes up.
- 1-2 days after you sent out a popular email newsletter to thousands of users.
- many international users visit the site - especially if pages are filled up with graphics and pictures. It may be necessary to mirror the site on servers in other regions of the world.
- during Christmas shopping.
The problem will be even worse if prices, syndicated content, etc. is requested from yet another slow server.
Ideal Response Times
The standard guidelines for response times are (Nielsen 1999, page 42):
- 0.1 second (one tenth of a second). Ideal response time. The user doesn't sense any interruption.
- 1 second. Highest acceptable response time. Download times above 1 second interrupt the user experience.
- 10 seconds. Unacceptable response time. The user experience is interrupted at an alarming high rate and the user is likely to leave the site or system.
These numbers are highly useful when planning server capacity.
References
Nielsen, Jakob (1999). Designing Web Usability. New Riders.
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Slow response time on Magasin.dk
Step 1: White page in 0.5-2 seconds.
Step 2: Some navigation is visible. "Text loading..." for another 0.5-1 second.
Step 3: "Text (still) loading...". Notice how the left margin jumped to the left.
Step 4: Product content loads as the last element. Ready to use after 5 seconds in total. Not good.
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