What cause the front loader to lift slow?

I got a 1996 New Holland 6610 tractor with a front end loader. The loader lift slow. Can raise the motor up as high it go & it will raise a little ffaster. It take about 25 sec. or more to lift it all the way at high idler. It will not raise the front tire off the ground. I done change the hydraulic fluid & fliter. I put 134 hydraulic fluid in it. It didn't help. There is a rattling noise in the transmission. It not to loud. Any suggestion. Thank you.