I also currently have a Slingbox Pro HD and have been (and continue to be) DISGUSTED by the lame-a** Mac support from Sling since DAY ONE and the utter MISNOMER of the Pro HD being either PRO *OR* HD (no HDMI and pathetic Mac support should warrant BIG RED warning notes on the specs and description, at the very least).
The Mac player ALWAYS lagged and never got any HD support and now they KILLED IT (to make matters worse, they are trying to route you to only use their web player, but the plugin download server has been dead for lord knows how long, but at least all day today). However, when the Mac player works, it works well for me even if sometimes grainy/analog looking (full screen mode). The only freeze(s) I ever had were due to the network/internet issues at the Sling home, or my remote end or somewhere in between. Another thing that might not be noticed, but will KILL SlingPlayer is when Time Machine runs its backup. It drove me nuts for about a month until I figured it out and now disable TM before/while using SlingPlayer!
Maybe check Activity Monitor (/Applications/Utilities/) and see if something is sucking-up cpu/RAM? Otherwise, check your network for a chatty/greedy device/connection (someone playing Call of Duty on PSN or XBox LIVE, or maybe streaming Netflix, etc.)?
On the other hand, I have had no troubles setting-up and viewing on my iPhone. However, it looks like Sling is going to further kick Mac/Apple users by making them pay $30 for the iPhone app and then another $30 if they also have an iPad. I've not seen such egregious double-dipping since George on Seinfeld!
Perhaps those who own/purchase the top-of-the-line Pro HD should get a discounted or free copy of any mobile app(s)?
Were it not for the iPhone app (pricing aside), and it working on 3G again (blame AT&T for that on/off/on nonsense!), I'd have eBay'ed mine long ago...