POTENTIAL REASONING:
In my experience with Modern Warfare 2, the program behind the game waits for certain conditions to be met before it sends off a packet across the network. For example, if your graphics card is taking too long to draw a frame, the program may wait for that frame to be finished before it sends or reads the next network packet. In addition, some drivers may choose to offload processing to the CPU if it detects that it is too much to finish in the allotted time.
I would ask: Are you the only one experiencing the lag or do others see you lag as well?
When my external harddisk takes a long time to write data, my network card stops transmitting until the other synchronized processes are finished.
This is best explained if you consider the case in which you are the host: as host, it is your responsibility to synchronize all the other players. However, if you are taking a long time to finish the computation processes, you will create lag even though your network connection is not being overloaded.
Ping is calculated not by the congestion on the network, but by how long it takes for you to respond to a packet. While the most noticeable source of intermittent delay is network congestion (bad connection), in your case it is probably processing delay, that is, your computer just can't keep up with the frequency of messages.
Hope this helps.
POTENTIAL SOLUTIONS:
If you are playing the native version of Starcraft II, I would suggest that you make sure your OSX is at version 10.6.3 (roll back or update as necessary) and try playing on the minimum settings. If you are playing on bootcamp with the PC version, I would suggest looking into overclocking your gfx card or updating the drivers. Bootcamp is known to underclock gpu's in order to maximize battery life.
So obviously selecting 1 is better than a group, but a little known fact:
Workers smart cast!
This means if you select a group of workers and try to build something, the nearest worker (and only him) will go to build the building. Additionally if you build more than one thing, the two nearest will go to do the task. This means you don't need to worry about selecting more than one (provided you don't issue any move orders), just select the group and build away!
Best Answer
During the replay you can see what they are building at every moment of the game by looking at the production overlay. You'd need a little more advanced knowledge of the build orders (which isn't hard to find on the internet) to determine what they are aiming for as a build order.