Hydrogen SEO vs Slim SEO: minimal footprint against a full dashboard
Slim SEO is lighter than Hydrogen SEO on nearly every cost we measured: +10.0 ms of PHP time against +12.9 ms, half the memory, a third of the package size and the smallest <head> growth in the field. Hydrogen SEO gives you more back: full sitemap lastmod coverage, more X tags, a settings surface you can actually configure, and AI features.
We build Hydrogen SEO. Slim SEO placed second overall in our benchmark, ahead of us, and it is the right choice for a lot of sites. The question is how much you want to see and control.
The numbers
| Measured | Hydrogen SEO 0.0.18 | Slim SEO 4.10.1 |
|---|---|---|
| Front-end PHP time added | +12.9 ms | +10 ms |
| Peak memory added | +4.5 MB | +2 MB |
| Database queries added | +1 | +2 |
| Hooks registered | +22 | +51 |
<head> bytes added | +4,259 | +1,649 |
| Meta description by default | Yes | Yes |
| JSON-LD types printed | 10 | 10 |
| Open Graph / X tags | 10 / 5 | 11 / 2 |
| Sitemap URLs listed | 72 | 73 |
| Sitemap image entries | 40 | 40 |
lastmod coverage | 72 / 72 | 50 / 73 |
| Custom database tables | 1 (2 with the 404 Monitor on) | 0 |
| Autoloaded options added | +0.4 KB | -0.2 KB |
| Plugin package size | 4.6 MB | 1.4 MB |
| Block editor assets | 2 files, 700.4 KB | 14 files, 187.7 KB |
| After deactivation | +0.2 KB left autoloading | Releases everything |
Measured on one WordPress 7.1 install (PHP 8.2, MySQL 8.0, Twenty Twenty-Five, 40 posts), each plugin alone with default settings, against a no-plugin baseline. Each column heading names the version that was measured. Bold marks the better number in each row. Full method: how we tested.
Hydrogen SEO's figures were measured on 0.0.18 and have not been re-run on later versions. The custom tables row is the exception: it is counted from the 0.0.31 source. A default install has one table, for the internal link counter. Switching on the 404 Monitor, which is off by default, creates a second for its log. Deleting the plugin removes both.
What the numbers mean
Both plugins are at the light end. Neither created custom tables in the benchmark; Hydrogen SEO has added tables since: 0.0.31 has one for link counts and creates another when the 404 Monitor is turned on, so Slim SEO now leads on this row. Slim SEO added +2.0 MB of memory against Hydrogen SEO's +4.5 MB, ships 1.4 MB against 4.6 MB, and was the only plugin whose autoloaded options came out slightly below baseline. Hydrogen SEO wins on queries (+1 against +2) and hooks (22 against 51). Both cleaned up almost completely on deactivation.
Slim SEO's <head> grew by just 1,649 bytes against Hydrogen SEO's 4,259, even though its schema payload is larger: 10 types in 3,828 bytes, against Hydrogen SEO's 10 types in 2,071. The difference is elsewhere in the head. Slim SEO printed 11 Open Graph tags but only 2 X tags, the thinnest X output of any plugin. Hydrogen SEO printed 10 and 5. If share cards on X matter to you, that is a real gap.
Sitemaps are close in shape and differ in one detail. Both include taxonomy archives and all 40 featured images (73 URLs for Slim SEO, 72 for Hydrogen SEO). Slim SEO left 23 of its 73 URLs without a lastmod; Hydrogen SEO stamped all 72.
The admin shows the philosophies most clearly. Slim SEO adds no admin menu items and loads 187.7 KB into the editor; it is designed to be close to invisible, with very little to configure. Hydrogen SEO adds two menus, a React dashboard with audits and a health score, and 700.4 KB of editor assets. Its AI layer and agent need a free account, and it is still Beta. If you want set-and-forget, Slim SEO does that better. If you want a place to see and fix issues, Hydrogen SEO is built for that.
Features: free, paid or not offered
| Feature | Hydrogen SEO 0.0.31 | Slim SEO 4.11.5 |
|---|---|---|
| Search engine verification codes | Free | — |
| IndexNow instant indexing | Free | — |
| Redirect manager | Free | Free |
| 404 monitor | Free | Free |
| SEO column on the Posts screen | Free | Free |
| SEO fields in Quick Edit | Free | Free |
| Attachment page redirects | Free | Free |
| Author and date archive controls | Free | — |
| RSS feed text | Free | — |
| max-snippet and preview controls | Free | — |
| Primary category | Free | Free |
| Internal link counts on the Posts screen | Free | — |
| Automatic image alt attributes | Free | Free |
| Table of contents block | Free | — |
| HTML sitemap | Free | — |
| Pillar or cornerstone content flag | Free | — |
| Crawl cleanup switches | Free | — |
| Video sitemap | Free | — |
| Google News sitemap | Free | Free |
| llms.txt | Free | — |
This table is not benchmark data. It is based on each plugin's WordPress.org listing, the code of its free version there, and its official documentation and pricing pages, checked on 12 November 2026, for the version in each column heading. Free means the feature is in the plugin's own free version. Paid means it needs that plugin's paid version or a paid add-on. No means the plugin's listing says it does not offer it. A dash means we could not confirm it either way; it does not mean the feature is missing. A row says a feature exists, not how complete it is. Plugins change, so check the plugin's own site before relying on a cell. If a cell is wrong, tell us and we will correct it.
Choose Hydrogen SEO if
- You want
lastmodon every sitemap URL, not two thirds of them. - You want fuller X card output (5 tags against 2).
- You want settings, audits and a health score in a dashboard.
- You want AI help or an agent for bulk SEO jobs, and you accept Beta.
Choose Slim SEO if
- You want the smallest footprint: 1.4 MB package, +1,649 head bytes, +2.0 MB memory.
- You want a plugin with no admin menus and almost nothing to configure.
- You want a lighter editor (187.7 KB against 700.4 KB).
- You do not need AI features and prefer defaults you never touch.
Switching between them
Hydrogen SEO's Tools → Import SEO Data detects Slim SEO and Slim SEO Pro and brings across settings, per-post fields and Slim SEO redirects, with a dry run first and an Undo afterwards. Features you had switched off in Slim SEO are treated as not in effect, the way Slim SEO treated them. See switch from another SEO plugin. Slim SEO also offers a migration tool for moving in from other plugins; check its documentation for which sources it covers.
Common questions
Is Slim SEO lighter than Hydrogen SEO?
Yes, on most costs. Slim SEO added +10.0 ms, +2.0 MB and a 1.4 MB package; Hydrogen SEO added +12.9 ms, +4.5 MB and 4.6 MB. Hydrogen SEO added one fewer query and fewer hooks.
Does Slim SEO have settings?
Very few, by design. It adds no admin menu items in our test and works mostly from defaults. Hydrogen SEO has a full settings dashboard with audits and a health score.
Which has the better sitemap?
They are close. Both include archives and 40 featured images. Hydrogen SEO stamped lastmod on all 72 URLs; Slim SEO left 23 of 73 without one.