Guaranteed Business Internet Speeds?? Not so much...

I was so excited to have an internet speed that allowed me to work seamlessly, for the first week or so. My guaranteed business internet speed of 150 mbps download, was tested this evening at 24.6 mbps download and 23.2 mbps upload (@ 5:59 pm on 5/10/18). Now compared to Comcast residential service (my old all time low residential speed of 2.3 mbps download) those speeds great for home internet. However, I run a software engineering company and require consistent internet speeds to accomodate our high traffic connectivity levels. I also noticed that the speeds dip in the evening and go back up late at night. I work with clients nationwide and need consistent speeds regardless of the time of day. Can someone tell me why this is happening?