Many plugins making WordPress run slow by design?

All the answers you see here are correct enough, but here's what probably (probably!) really matters:

If your multiple plug-ins use code redundantly, they'll be worse, performance-wise, than one plug in with all the functions and doesn't repeat itself.

This is actually WordPress'argument for "why Jetpack isn't a resource hog"; they wrote really clean common code and it loads once instead of (potentially, right now, in Jetpack, 36 times).

And Jetpack IS kind of a resource hog, but their argument is valid in its way.

There's the closest thing you'll get to an "answer"; if you split your big plug-in later and call a library containing common code just once you will have very little performance hit.

You know ... unless something else is wrong ;-)

Having two or twenty plugins does not make difference because of the number of plugins itself. Maybe you can notice difference from one plugin to 1000 thousand, but not from 2 to 20. What the plugins do is what makes difference. Having 50 light plugins can be fast, while having one single plugin doing a heavy job or really poorly coded can be quite slow.

So, there is two key things to take into consideration:

• What plugin does
• How good or bad the plugin is coded

That's beign said, if you you split 1 plugin into, let's say, 10 to do the same job, the difference is 10 include statements instead of 1, so there is impact on performance but so minimun that you won't notice it. Splitting one fast plugin into a few ones will continue being fast, splitting one slow plugin into few ones will continue being slow.

I would be wary of things you read - e.g. there's a common myth that "having lots of plugins" is a bad thing, whereas of course what matters is whether they're well written or not. The time to load a large number of very well written plugins is neglible, whereas a line of bad code in one can easily screw things up for everything else.

There might be a case to split a plugin if it makes sense structurally, and you are likely to be able to reuse the individual plugins elsewhere, but concentrate on improving the quality of your own code first:

• are you using classes (correctly)?
• are any of your classes or methods becoming unmanageable (e.g. cyclomatic complexity) - use a tool like PHP Mess Detector
• are you making common mistakes? try PHP Code Sniffer or JS Hint
• are you using the database correctly/efficiently? e.g. don't use query_posts ever
• are you following the coding standards
• is your code well documented? PHPDoc

Make iterative improvements.
Find a good IDE.
Refactor often.