Speed of a website matters !
You’re using a web application, it takes 5-10 sec to load each page or to complete ajax requests. How does it feels ? Awful, isnt’ it ? You will lose up your patience and will switch to a suitable alternative.
Similarly, you own a SaaS application with a good user interface and many useful functionalities. A end-user sign-up and found it to be slow enough for his use. Every page keeps on loading for 5-10 seconds.
Will she come back again to your ? Will she upgrade her package from free to a paid member ?
This is why speed of a website matters.