Using the 'G' key to move my character in solid mode moves fine. However in material/render preview the character moves slow. I've hidden a few things from the render but no luck. Any idea why? Thanks
If you are using cycles, and facing this issue, then most probably, this is the reason. Everytime you move around the scene OR move an object around in the scene, the lighting has to be recalculated accordingly.
Now, say you are using Eevee. Then the issue might be that you are making changes in the pose mode and there might be modifiers after the armature deform modifier which can be causing the lag.
Since not much details have been provided in the question I can't think of any other reason.