A significant number of themes that are available for WordPress is marketed as “fast themes”. However, depending on the amount of content you want to display per page, some themes can certainly be more attractive than others, as they use fewer client-server requests, and are able to process and display the content faster. Other themes can be tempting as they offer a higher quality of visual presentation, but with additional elements which use extra scripts and coding that will create overhead client-server communication.
After applying a theme and some content, you should use a number of different software tools to make sure your website is truly “fast”. For example, Pingdom Tools, Webpagetest … etc. The most important parameter is the number of HTTP Requests, which should be low. Currently, search engines are penalizing website ranking for websites that load longer than 4 seconds, so make sure that your websites homepage, and other pages loading times, are under this mark. Number and size of elements that display a picture can add significant loading time to the webpage so be sure your overall any single page size is low.
The framework on which the theme was built can give some insight to the performance it offers. Most themes built on Bootstrap are light with coding and scripts and can be considered when looking for a fast theme. Another lightweight but somewhat older framework is Wonderflux.
Many of us will awe at the Envato multipurpose themes which offer the rich visual presentation with a number of widgets and other elements, though it must be noted that those are mostly heavily bloated to be able to support a wide range of integrated elements, and will require thorough testing to make sure the theme is not crippling your website.
You can look for premium fast WordPress themes on ThemeForest, with rating and reviews, that can help you choose one. Some of my personal favorites are Invictus (multipurpose theme) and Atisomya (clear and beautiful).
At this day and age, when everyone is in a hurry, people are not patient enough to wait for a website to load but will instead go for the BACK button, and you will be losing visitors and traffic if you neglect that fact. Do not let a bloated multipurpose theme slow your website.
Ultimately, if you really are in no position to change your WordPress theme, then your only option is to get a high-quality WordPress hosting service, that will compensate your slow website with a high-performance hosting.