OP here. Readers of your resume, and ultimately hiring managers, will make assessments of your overall capabilities based on the type of achievements you use in your resume.

If a senior engineer cited an improved loading time that could be googled in 5 minutes as one of the 4 or 5 accomplishments in her or his two years at a company, they'd be sending a bad signal about the type and scale of achievements they're capable of. Hiring managers will be looking for something more level-appropriate and that indicates a relevant level of skill.