Intermittent, but significant, server lag

The app I have hosted on AWS has lately been experiencing intermittent, but significant lag. Simple requests that are just flipping a boolean flag on a table from true to false usually take less than a second. But lately these requests wind up taking 30-50 seconds. More complex requests in the application are getting hit even harder by this lag.

I inherited this application from the previous maintainer, and I'm not overly familiar with debugging these kinds of performance/server issues on AWS. I've tried rebooting the instance, and that doesn't appear to have helped. Would appreciate any suggestions or guidance on what I should be looking at to try and resolve this issue.