Most people choose a theme by looking at the demo. But every demo looks good: it is a professional layout with well-chosen photography and content written to fit the design exactly. Your real content will not look like that.
What you should check before buying is elsewhere. There are three things, and all of them take five minutes.
1. When was it last updated?
Theme listings show a last-updated date. Drop anything without an update in the last six months. WordPress core keeps moving, and a theme that does not follow eventually breaks. Worse, a security issue found in it will not be fixed.
On the same page, check whether support threads are being answered. Recent questions sitting unanswered for months usually mean the author has quietly moved on.
2. Is the demo itself fast?
Paste the demo URL into a speed testing tool. The demo is the author’s best-case setup, so if it is slow there, your site will certainly be slower.
Multipurpose themes packed with sliders, animations and icon packs frequently load those assets whether you use the features or not. If drawing one page pulls down twenty scripts, your visitors pay that cost on every visit.
3. What are you actually buying?
A premium theme price usually covers one year of updates and support. After that you choose whether to renew, and if you do not, updates stop. Plenty of owners discover this in year two while wondering why nothing updates.
Check the bundled plugins too. When a theme ships commercial plugins, those plugins are updated only through the theme author. If the author slows down, they sit there carrying whatever vulnerabilities turn up.
How theme and plugin choices affect performance is covered further in the Themes & plugins archive, and if you are already running on a heavy theme, our process page shows how the improvement work actually runs.
Next part
With a theme chosen, plugins are next. To give away the conclusion: you need about five of them to start.