Q: Why mac mirroring is not perfect like iphone, ipod or ipad mirroring??

If you are watching video it is better to use basic airplay as that will simply push the video to the ATV.

Mac mirroring is more intensive, since it is pushing out the entire display in realtime. If there is lag that will be caused by the network. Internet connection isn't relevant as this is done over the local network. 

Make sure no other activity is occurring (other streaming, growing, network backups etc)

If on wifi try ethernet

Test on another network

I definitely agree. What you see with the iPhone or iPad is probably basic AirPlay, not mirroring. You can tell because the video player window turns gray when using basic AirPlay.

You can use basic airplay within iTunes and the subtitles will show just fine. As I stated before the internet connection is irrelevant because this is over the local network. The local network is where the issue lies, so if you want to continue using mirroring you will need to troubleshoot your network.

You can also see a current status of the network by getting a report from istumbler, netstumbler or similar. Look for signal strength, noise, nearby networks.

The lag with mirroring is partly due to the extra processing required. As the screen is updated in the computer frame buffer, mirroring uses magic code running in the graphics chip to generate compressed H.264 video, which is then transmitted to ATV over the network. That complex process cannot be instantaneous, while the audio can be transmitted with little delay.

I also have this problem, but I disagree with "the local network being the problem" answer.  It has streamed fine for several months, and then one day an update came along and things were never the same.  Good luck and I will post if I find a solution.

- Tired of Apple*sauce

Network issues can come at any time without any changes done by the user. Mirroring is done over the local network and any issue will require proper troubleshooting. There is no such widespread issue caused by an update.

It's the same processing in both cases, but the phone starts with a smaller screen with fewer pixels. Beyond that, I don't know. Maybe your 4-year old computer doesn't have a powerful enough graphics processor. This all depends on hardware capability, it can't be fixed by better software.

Yes I do know what you mean, it will come down to how it's encoded. There is a big difference between mirroring a phone and a full computer display in real time, it requires more resources from the network and if there is lag it's because yours has an issue handling the load.