Low hot water pressure
So you have low flow rate out all the hot water lines. This indicates a restriction in the hot water system which could be in a pipe or in a valve. The latter is more likely in my experience. 

The restriction could be in the cold water line that feeds the water heater or it could be in the hot water line before the dividing into the various usually smaller lines that go to different areas.

All systems have a cut-off valve on the cold water feed to the water heater. Sometimes these valves develop a restriction, if they are of the old gate valve type. I am not aware of quarter-turn ball valves developing restrictions.  

Some systems in addition have a cut-off valve on the hot side of the water heater. This valve could also develop a restriction. Do you have one of these?

Did you recently operate one of these valves before this low flow condition has occurred?