Slow loading speeds deter users from engaging your website

Development tools

All Online Tools
Bootsnipp
Color tools
CSS Tricks
Customize Bootstrap
Favicon generator Web Dev
Free Web tools
Frontendmasters
Increase Website Traffic
Painter
XML-Sitemaps.com
ThoughtCo
Webmaster tips

Web tests

Broken Links Checker
Can I use?
Contrast/ratio Test
Mobile friendly test
MobiReady
Nibbler
PageSpeed Insights
Search Engine alternatives
Siteliner
WAVE
Website Speed Test & Performance
Webmaster tools
Yellow Lab Tools

SEO test tools

Backlinko.com
BrowSEO.net
Chrome Dev Tools
Chrome extension
Google Web.dev
Google Search-Console
Keyword Mapping for SEO
Learn SEO
SEO Power Suite
Tap Target Tester
Totheweb.com
Website SEO checker
What is an SEO title