I have a directory with roughly 100000 files in it, and I want to perform some function on all files beginning with a specified string, which may match tens of thousands of
Use find with a wildcard:
find . -name 'mystring*'
If you want to restrict your search only to files you should consider to use -type f in your search
try to use also -iname for case-insensitive search
Example:
find /path -iname 'yourstring*' -type f
You could also perform some operations on results without pipe sign or xargs
Example:
Search for files and show their size in MB
find /path -iname 'yourstring*' -type f -exec du -sm {} \;
ls | grep "^abc"
will give you all files beginning (which is what the OP specifically required) with the substringabc.
It operates only on the current directory whereas find operates recursively into sub folders.
To use find for only files starting with your string try
find . -name 'abc'*