Some of the hosting providers out there are infamous for overselling who try their best to stuff in as many users (websites) as possible into a single web hosting server. High server load is an indicator of how your server is performing and whether it is laboring too much thus jeopardizing the performance of your websites. You can get to know the average load in the last 15 minutes of your server by the simple Linux command below (via SSH):
Which will typically return a line of data similar to this:
21:39:33 up 10:45, 3 users, load average: 4.46, 3.92, 3.64
That says there are currently 3 users logged on and the load average of this server in the last minute, last 5 minutes and last 15 minutes are 4.46, 3.92 and 3.64. These figures represent the number of runnable processes at the same time on average for the CPUs (processor) to process. Combined with number of processors of the server, you may know how many processes are being processed by any single CPU.
Considering the fact that any CPU can only take on one process at any given time, there will possibly be processes waiting in the queue – meaning server is overloaded. Therefore, if the number of processors of your hosting server is 4, in the last minute, it is overloaded by ( 4.46 / 4 ) – 100% = 11.5%.