It depends on the program. There is actually no universal answer to your question.
CLI-parsing is entirely done by the program, often using
getopts, but not always. For example, the
find command arguments all use the single-dash prefix but none are actually groupable. (This convention has been discouraged for quite awhile.)
Many modern programs have done away with the single-dash option prefix entirely. It ultimately takes a lot of code (such as encapsulated in getopt/getopts) to handle all the permutations and is the antithesis of "self documenting code".
The double-dash prefix, which never allows grouping, is far simpler to parse and is far more self-documenting since it promotes spelling out the option, or at least a common abbreviation for it.
So too, the greater number of keystrokes is not really an issue anymore, as all common shells support pressing the up-arrow to present previous command lines and Ctrl-r to search those previous lines, which can then be edited. Thus, the full command string only has to be entered once. In addition, most command line work is done through terminal programs these days so mouse copy-n-paste of the
history command output is also available.