Do you know how many shared hosting account are usually hosted on a shared hosting server ?
I have worked with a few hosting companies, most of them hosting between 200 ~ 250 accounts per server (Yes, it is 250 !).
So, you may ask, having this many accounts on a server, isn’t it will slow down the website access ? The answer is , it depends on the number of popular website on the server and the resources usage like memory, hard disk speed and CPU usage. In most cases, 95% of the domains are idle most of time, only a few domains keeps active all the time.
If you are using shared hosting and experienced slow access regulary, it is most likely that there are more active websites on the server. Contact your host and request moving your account to a lower usage server, it is their responsibility.