When a WordPress site slows down, developers usually open Query Monitor first and reach for a caching plugin such as WP Rocket next. Both are excellent at what they do, but they do different jobs. Flux Speed answers a third question: why was the site slow last night? This post compares what each tool measures, what it fixes and when you need which.
The short answer
- Query Monitor is free and very detailed, but it only shows the page you are looking at and keeps no history.
- WP Rocket is a paid caching and optimisation plugin: page cache, lazy loading, delayed JS, unused CSS removal. It speeds things up rather than measuring them.
- Flux Speed records slow and failing requests with their queries, errors and outgoing HTTP calls, shows server health and offers safe clean-ups. It is not a caching plugin.
- In practice: Flux Speed to measure; your server’s cache (such as LiteSpeed Cache) or WP Rocket to speed pages up.
Comparison table
| Query Monitor / WP Rocket | Flux Speed | |
|---|---|---|
| What it does | Query Monitor measures (live). WP Rocket speeds up (cache and optimisation). | Measures and records; maintains the server and database. |
| History | None in Query Monitor; the data is gone when the page closes. | Slow and failing requests are saved; p95, average and longest time for the last hour, day or week. |
| Developer panel | Very detailed in Query Monitor: queries, hooks, HTTP, template. | Admin-bar summary and bottom panel: queries, PHP errors, HTTP calls, template, hooks, timeline. |
| Page cache | In WP Rocket. | Not available. |
| Server health | Not available. | CPU load, RAM, disk, PHP memory, OPcache, cron status and suggestions. |
| Clean-ups | WP Rocket has database clean-up. | Expired transients, auto-drafts, spam comments, table optimisation, autoload analysis. |
| Price | Query Monitor is free. WP Rocket has no free version, with a 14-day trial. | Part of FluxWP. |
| Load on shared hosting | Query Monitor runs only for the session it is open in. | Visitors trigger only cheap hooks; query capture runs on sampled requests and for logged-in staff. |
The real problem: slow requests happen while nobody is watching
On shared hosting the slow moment is usually one nobody saw: a cron job, an outside API answering late, or one plugin running hundreds of queries on one page. Query Monitor cannot see that moment, because it only inspects the page that is open. Flux Speed saves slow and failing requests with their queries, errors and HTTP calls; click a row later and you get that request’s full panel. Emails, phone numbers, IBANs and tokens are masked before anything is stored.
Measuring and speeding up are different jobs
WP Rocket’s job is to get the page to the visitor faster: caching, lazy loading, delaying JS and removing unused CSS. Flux Speed does none of that; it shows you what is slow. A cache can hide a problem from visitors, but the admin, the checkout and logged-in users are not cached, and there you need the cause. Our Core Web Vitals guide covers the visitor-side metrics.
One System Check
Flux Speed collects the checks of every FluxWP plugin on one screen: is cron running, is autoload large, is the disk almost full, is OPcache off. Problems that can be fixed have one-click fix buttons.
When a competitor is the better choice
- You want to dissect one page while developing. Query Monitor is the standard tool for that, and free.
- Your server has no cache and the pages need to be faster now. WP Rocket works out of the box. On LiteSpeed servers, LiteSpeed Cache is a free alternative.
- Flux Speed does not replace them; it runs next to them. Use it alongside your caching plugin.
Frequently asked questions
Does Flux Speed slow the site down?
Visitor requests only trigger cheap hooks such as errors, HTTP calls and timing. Detailed query capture runs on sampled requests and for logged-in staff. Each record is written with a single INSERT at the end of the request.
Can I use it with Query Monitor?
Yes, but their developer panels do the same job, so one is usually enough.
Bottom line
Query Monitor is great for live inspection and WP Rocket for speeding pages up. To catch the slowness that happens while you are not looking, see how the server is doing and answer “why was it slow yesterday?”, Flux Speed fills the gap.
Sources
- WP Rocket pricing (plans and features)
