I have a plan to start a website with different topics and tools. And my doubt here is... is it okay to present all these topics on a single domain (example.com), or is it better to make some subdomain for each category?
In this case, I want to consider the reliability and loading speed.
From a speed point of view definitely 1 domain. This results in fewer DNS lookups and the ability to reuse connections which speed things up.
Reliability is a complex issue. Its likely better solved with good DNS infrastructure and hosting - possibly leveraging a reverse proxy / caching system like Cloudflare to boost speed and resiliency - and/or, depending in the content, maybe having multiple servers serving the content.
The only thing I see multiple domains providing is resiliency against the domain name being taken away - if you are not doing stuff governments don't approve of, long term registrations through reputable registrars (and ysing 2fa) can largely offset this risk.
