The first thing you notice when a site starts working is that it gets slower. A page that was fine yesterday arrives a beat late today, and the admin screens feel worse still. Most owners reach straight for a bigger server, and usually that is not the cause.
WordPress is a well-built tool. Slowing down is not the tool hitting a limit — it is a configuration that has not kept pace with the site’s growth. The cause is nearly always one of three things, and checking them in order usually settles it within half an hour.
There is more than one kind of slow
Start by separating two very different symptoms hiding inside the word “slow”.
Open your browser’s network panel and reload the page once. Simply seeing whether the first request (the document) is slow, or whether it is the images and scripts that follow, tells you which side you are on. Skip that step and you will happily optimise the wrong thing.
Check 1 — what was added recently?
Slowdowns tend to follow a change. Think back over the last few weeks: a newly installed plugin, a new third-party script (chat widget, ads, analytics, fonts), a large new page.
Third-party scripts are the quiet culprit here. However fast your server is, if that script does not answer, the page stops where it stands. Switching them off one at a time and watching how the page feels is the quickest possible test.
Check 2 — have the pages got heavier?
Pages grow with the content on them. A few photographs uploaded at full camera resolution are enough to change how a page feels on their own. Camera originals are far larger than anything the screen needs, and resizing before uploading is the cheapest performance work there is.
Look at listing pages too. Loading dozens of posts into one screen means the database does that much work every time. Simply showing fewer per page often helps more than you would expect.
Check 3 — is it rebuilding everything on every visit?
The third cause is the most common and gives the largest improvement. Left at its defaults, WordPress assembles the page fresh for every visitor. With few visitors that is fine; as they multiply, you are repeating identical work thousands of times.
Removing that repetition is what caching does, and it is normally where the biggest single gain comes from — which is the subject of the next part.
Upgrading the server comes last
A bigger machine will help. But it is paying more to absorb the same waste, so the same problem returns at the next growth step. Clear the three above first; if you still need more capacity after that, then buy it.
Articles that work through performance layer by layer live in the performance archive, and if you would rather hand over diagnosis through to caching as one job, our optimization program includes before-and-after measurement.
Next part
Next comes caching. “Turn on a cache and it gets faster” is only half true — caching is not one thing but several layers with different jobs, and knowing the difference is what stops you stacking them wrongly.