slow turn over

1977 kz650b battery is fully charged but starter motor is slow to turn the engine over. starts ok on the kick starter. put the meter on the battery and pressed the start button and voltage drops to 6 volts. does this indicate the battery is faulty or the starter motor is at fault. any input is appreciated. cheers guys.
 

You measure 6V at the battery terminals under load?  If you measure that at the battery terminals, your battery very likely has an internal fault and you need a new battery.  

I think DOHC is on it. You can jump your bike with your car but do not start the car. I would disconnect the bike battery. If the charging see a 6 volt battery is gonna hit it with a spike voltage, I’ve had it happen and it blew every light bulb on the bike. 

You can also take the battery to your local automotive store and they can test it. Charge it full first.

Could also be starter brushes or worn bushings. I almost bought a battery but it was the starter.
Steve

Simplest/cheapest solution would be a bad ground. Check that first. 

wrote:

Simplest/cheapest solution would be a bad ground. Check that first. 

A bad ground would reduce the load on the battery, making it even less likely that one would measure 6V at the terminals.  

Steve's suggestion about the starter motor would increase the load and possibly make it droop more, but 6v is very low.

I like the suggestion of substituting a car battery as a test.  If it starts normally from another battery then the existing battery is probably the issue.

Rule out the battery/connections first. If the starter drags with another battery it could be bad. A larger battery could make it work but it won't spin as normal. I'll be buying brushes for my starter being the bushings are good. 
Look for the bike you have. 
Steve
ricksmotorsportelectrics.com/Starter-Motor-Brushes-70_103

Check the acid level of the battery cells!!! One dry or half full cell(s) can effect a huge voltage/current drop if you use the starter motor.

tried to start using the car battery and starter motor is still slow so it looks like the brushes in the starter motor are worn 
so i will pull the starter out and give it a good dose of looking at, cheers for the input guys.

Be sure the bushings aren't worn out. If they are the brushes might work but not for long. I use to buy the rebuild kits but they are hard to find now. I wound up buying a low mile starter mainly for parts. When I get my new brushes I'll take the best parts from both and make one good one like I did the last time. 
Steve