Writing to plist file multiple preferences same time
Mac's default command is not designed to work like that.  

You will see a substantial speedup by having Applescript make (or call) one shell script to issue all 50 commands. You can wrap both versions of the script in a date call or use the time function to time the shell script if you are not sure where the delay lies on your Mac.

The vast majority of the delay will likely be Applescript looping and then forking each an environment for each script to be executed. By writing one script, not only will the startup be reduced to 1/50 of the time, it will likely execute far faster overall.

If you really wanted to, you could write yourself a function to parse multiple inputs; but it's probably easier for you just to have 50 lines of shell script.