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The importance of performance

Concept · Chapter 10

The idea in brief

Section titled “The idea in brief”
  • Performance is the responsiveness of the operations an application performs. Users expect fast response times regardless of location or device.
  • Correctness first: software must serve its functional purpose, be reliable, and be usable. Once those are met, performance becomes highly important — speed can’t rescue a broken app.
  • Performance is a quality attribute, not an afterthought. It belongs throughout the whole application life cycle. See Quality attributes.

Performance affects user experience — and the bottom line

Section titled “Performance affects user experience — and the bottom line”
  • Speed is a major driver of overall UX and user satisfaction.
  • It moves the organization’s bottom line: customers gained/lost on customer-facing sites, productivity gained/lost on enterprise apps.
  • On web/mobile, slow page load is a major cause of page abandonment — if a page is slow, users leave.

Bounce rate

Section titled “Bounce rate”
  • A bounce is a single-page session where the user leaves without visiting any other page (a.k.a. exit rate — the percentage of users who bounce).
  • As page load time rises, bounce rate rises.
  • Bounce actions: closing the tab/window, clicking away to another site, hitting back, typing a new URL, or a session timeout.

Conversion rate

Section titled “Conversion rate”
  • The conversion rate is the percentage of visitors who take the desired action (place an order, register, download, subscribe).
  • Poor performance lowers conversion — slow sites push users elsewhere.

Performance is a requirement

Section titled “Performance is a requirement”
  • Speed is a feature: “if it is not fast enough, then it is not good enough.”
  • Like any requirement (see Requirements engineering), a performance requirement must be:
    • Unambiguous — not “loads in a timely manner” but “loads within two seconds.”
    • Measurable — specific values/limits.
    • Testable — you can assert whether it was met.
  • Because it’s a requirement, it should have tests. Measure execution time and assert against a time limit. Performance tests run less often than unit tests but should be easy to run regularly.

Page speed affects search rankings

Section titled “Page speed affects search rankings”
  • Page speed is a factor in Google’s mobile search ranking.
  • Currently it mainly penalizes the slowest pages, but it signals how much weight Google places on web performance — for customer-facing sites, don’t let slowness hurt your ranking.
Section titled “Related concepts”

Citations

Section titled “Citations”
  • Software Architect’s Handbook (Packt, 2018), Ch.10 “The importance of performance”, pp. 734-739.