Ive looked around and also asked in game and ive never been able to find out a definite answer. Sometime I have issues with blocks not loading or them loading EXTREMELY slow is this due to my pc/internet connection or just overall server lag?

Think server lag

When you destroy a block and it comes back

This is really annoying!!!

I'm more annoyed with all the people teleporting everywhere when they are actually just walking. Also, the mobs are REALLY laggy...

I've noticed that with the mobs and blocks too. I know it's not my internet connection as I have an average ping of about 105 to the server. And it is DEFENITALY not my PC (6 core i7 with 12GB of RAM and NVIDIA GTX580 GPU). Is this just due to the traffic on the server or a known issue with Minecraft itself?

I have the same issues with mobs and blocks loading.