If you have multiple WordPress sites, how should you set that up?
Some people say to install each site on a separate installation of WordPress, keep each site lean and fast. Some people say WordPress in Multi-Site mode is fewer files, less administration, just as fast. Which is it?
How does the use for your entire hosting account compare?
Overall, keep your installed themes and plugins to the minimum you need; keep your unused themes and plugins “elsewhere”, for security and speed. In Multi-Site, only “network enable” the plugins you actually need on all or almost all sites. But does using single-site or multi-site change overall speed or efficiency or storage?
Company A with 30 sites, installed as 30 separate WordPress sites (on one hosting account, it’s easier and much less expensive than multiple hosting accounts). Company B with identical 30 sites, on WordPress Multi-Site. Same content, same themes, same plugins.
You must install whichever one you pick, on a hosting account able to handle the load of all your sites, including any surges in traffic (for example, your site gets mentioned in any major TV show or web site).
Each has 29 sites that load once and 1 site that loads 500 times (per week, per minute, doesn’t matter).
The company uses pretty much the same plugins for each site, for example: backup, FaceBook, anti-spam, form processing, video player, etc. The busy site also uses a huge membership plugin and a huge store shopping cart order processing plugin.
Cost to the hosting account for Company A: 529 page loads.
Cost to the hosting account for Company B: 529 page loads.
MySQL use for each company: almost identical, see below.
WordPress core file requests for each company: identical.
WordPress theme file requests for each company, if use the Company Theme: identical.
WordPress theme file requests for each company, if each site uses a different theme: identical.
WordPress plugin file requests for each company: almost identical, see below.
It doesn’t matter at all whether using WordPress or WordPress Multi-Site, it is the same core code, the same theme code, the same plugin code, the same database use. Each individual site, each page, has different file and database usage, but the overall usage is virtually the same single-site or multi-site.
Server usage: The WordPress Multi-Site files are identical (one installation), the WordPress files are copies (30 separate installations). Server caching is more likely to serve cached files for Multi-Site.
If the company uses the same theme for all its sites, on Multi-Site the theme files are more likely to be cached; if different theme for each site, cache use is identical.
Ditto for the collection of plugins.
Using a Content Delivery Network, such as CloudFlare (but check with your hosting provider, they may have tighter integration with Cloudflare), makes this caching difference even smaller.
Database use: Database usage is identical, except that on Multi-Site, each site has a unique ID, which takes one extra database query per page load. (Each WordPress page takes dozens to hundreds of database queries.)
WordPress uses record locking (meaning changes only tie up that one record, not the entire database table), and each site has its own set of database tables. MySQL has virtually identical performance whether using 30 databases or 1 database with 30 times as many tables.
A page load in WordPress needs dozens to hundreds of database queries. (And dozens of files, and dozens of HTML sub-requests; optimize these, then optimize the most frequent and/or slowest database queries; the Multi-Site overhead is very likely minimal.)
Plugin use: Identical except that on Multi-Site, mapping of somedomain.com to site ID 27, would be done using the Domain Mapping plugin. Only plugins that are Network Enabled are loaded for every site on the network.
Conclusion: Negligible difference in page load time, database use, memory use, disk use. Disk storage for MultiSite is lower, since only one copy of WordPress core files (about 70MB per installation, with duplicated plugins perhaps 100MB, also negligible for most hosting accounts).