This is a spinoff of bug#51622. While looking at the performance of 
'abbreviate-file-name' for Tramp files, I noticed that 
'file-name-case-insensitive-p' was taking up a significant percentage of 
the execution time. I dug into this and found two main hot spots:

1) 'tramp-handle-file-name-case-insensitive-p' calling 'file-remote-p' 
and 'expand-file-name'

Since 'file-remote-p' only needed to check whether a connection was 
already established, it could be replaced with this (thanks to Michael 
Albinus for the pointer):

   (let ((non-essential t)) (tramp-connectable-p v))

'expand-file-name' also had room for a small optimization, since it 
previously called 'tramp-connectable-p' (which dissects the file if it's 
not already) and then 'with-parsed-tramp-file-name' (which dissects it 
again). I reversed the order so now there's one fewer dissection, and 
it's a bit faster.

2) Potential handlers in 'tramp-find-foreign-file-name-handler' each 
dissect the file name

Most Tramp methods have a 'tramp-FOO-file-name-p', and most of *those* 
take a file name string and dissect it. This is a lot of duplicated 
effort, so I modified 'tramp-find-foreign-file-name-handler' to pass the 
dissected file name to any of the functions that support it (this is 
indicated by an 'accepts-vec' property on the function). This probably 
warrants some documentation (at least a NEWS entry), but I wanted to be 
sure the strategy made sense before I wrote any docs.

With these changes combined, I see the following results (testing with 
the sshx method connecting to localhost on a GNU/Linux system):

* 'file-name-case-insensitive-p':
   3.5x faster, now 583μs per call
* 'tramp-handle-file-name-case-insensitive-p':
   4.5x faster, now 281μs per call
* 'tramp-find-foreign-file-name-handler':
   5.2x faster, now 45μs per call

In addition to the patches, I've attached the benchmark script that 
generated these results as well as the raw data.