You'd really have to post code to get a specific answer.
Almost everything you list
might be faster on the commandline, or might not.
As always, its best to benchmark where the real problems are using a benchmark class, and determine what portions you need to improve.
Then, take that section of code, and ask for help with it.
You never know what little optimizations you havent thought of that could save you almost 50% -
it happened to me!
But beyond that, if you truly want to try moving those sections of code to the shell, then do so - one at a time, and benchmark the time in php, and then running in exec by php.
Also, since you mentioned dialup users, are you using output buffering and gzip compression? Both help immensely with dialup users.
Also, use HTML-compliant code, which helps the display render faster.
There are many suggestions that can be made on how to make things "go smoother" for you, but you've given very little information and asked a very specific question.
The answer is
maybe. Your best
specific answer will come from specific code, benchmarks, and testing.