We have become a very impatient society and many internet users will only wait a matter of seconds before leaving your website and going elsewhere to find what they are looking for which makes the speed of a website more important than it has ever been.
Back in the early days of the internet it was common to wait for a website to load, people where using slow dial up modems and the internet was slow but as speeds increased over the years and broadband was introduced, load times became quicker and developers decided to build more complex, code heavy website utilising large images, streaming video etc.
Nowadays we expect a website to load instantly and when it doesn't people assume its offline so click away which is a big no no if you use your company website to sell products or services.
Google loves a quick website and while its not a major ranking factor it is one, so you are not just trying to keep your clients happy.. The internet is vast, and there's no telling how often Google will crawl your web site so you want them to get as much as they can the first time around.
A good resource is tools.pingdom.com which allows you to enter the URL of your website and check load times, it's free to use and gives you advice on how you can speed up your website.
You website should load within 2 to 3 seconds to be considered quick loading so if your seeing results upwards of this then you may have a problem and need to go back to your web designer to get it fixed.
It's my opinion that all web designers or design agencies should be checking the speed of your website before it's launched, after all your have paid them for a service which should include this important step. Unfortunately there are a growing number of amateur designers and design agencies around who are basically just installing free web site builders or pre build templates without having enough knowledge of the industry they are working in.
Open source software and web builders are notoriously slow with many taking between 4 and 10 seconds to fully load, which in website terms is a lifetime. If you mention slow loading speeds to your designer the usually answer is "well I didn't write it" which says it all really.
Popular DIY platforms will offer add-ons known as plugins that falsely claim to speed up the platform which is like adding more cargo to an overloaded truck with the intention of making is go faster.
In summary, the speed of you website is almost as important as its content so when launching a new site make sure that speed tests are done and that you're happy with the speed before finalising with the designer.