@Mogster is right.
While you won't be making a "custom client" per se, when you [freeze items, it amends the .mul files and does have to be redistributed to your playerbase in order for them to see the changes.
Because Freezing saves to a file that is on the user's own computer, they don't have to redownload information from the server as they are playing the game, which is what decreases "lag".
You can also lock down items, which saves to the server's save files, but I don't think this does much in the way of relieving lagtime at all.
However, I'm not exactly sure how
@Praxiiz 's
Ultima Live streamer works, but it should have the potential to alleviate some lag as, I believe, it pulls in smaller blocks of data by area surrounding the character. Because Ultima Live also allows dynamic content, it may not require saves to the mul files. I'm not entirely sure, but it's something for you to look into.