Install P3 (Plugin Performance Profiler) plugin. Start test.

This plugin creates a profile of your WordPress site's plugins' performance by measuring their impact on your site's load time. Often times, WordPress sites load slowly because of poorly configured plugins or because there are so many of them. By using the P3 plugin, you can narrow down anything causing slowness on your site.

Disable the plugins which are using high resources and your problem will get solved.