It's normal with all the ports closed that the ram moves very slow and stiff, it shouldn't move at all even.
But I guess there are some small losses in unpressurised Burkerts.

Now, how did you connect it all up? Any restriction in flow will make a large ram like that move slow.
The ports in the ram are 1/2bsp.
If you use 2 Burkerts 5404's to feed the ram, there should be 2 hoses or pipes between those and the buffertank. Internal diameter of hose/pipe should be at least 12mm.

If these conditions are fullfilled, there is another possible reason to have a "limp ram". The shuttles not moving in the main valves.
Option 1)
The pressure is over 50 bar, some Burkerts need more than 24V to open the solenoid fast enough to have the shuttle move, otherwise it's just the 2mm assist hole in the Burkert that feeds the ram.
2) Foreing matter in the valves ( swarf or sticky iccy stuff)