Hi,
this is really strange, because the fast moves are working…
Let’s see… can you manually move all axis without any problem? can they move freely without a lot of force? If not, try to make them as freely movable as possible.
And, perhaps try a new firmware to fix it. I recommend Skynet3D which is Marlin pre-configured for the Anet A8. Try to upload the software to your Anet and try it again to move it.