You did just use the buzzword “mobile first” to describe the design of your latest website. But did you really do it right? Try my 5-second mobility test (ok, it could take a lot more seconds):
Open up Chrome Developer Tools (cmd-alt-I), click the Network tab and make sure the Disable cache checkbox is checked. Adjust the browser width to mobile size. Then, reload the page. Wait for the page to download. Really, wait. See the bottom status bar and especially the amount of data transferred.
If the amount of bytes transferred is something like less than a megabyte, you are ok, but obviously the less the better. If, on the other hand, the amount is several megabytes, then… well, you should reconsider your original statement about being mobile first – I’m not sure that would be first anything.
Sure, download size is just one factor of the whole page speed experience, but remember that this is just a quick test I like. You might like to use Google PageSpeed Insights instead, for example.
My point is this:
Whatever design approach you use, it is not really worth a damn unless you make sure the approach is actually executed. It really is the only way.