I can't answer your question about performance because I don't own a S4. However, I have both my Note 4 and Note 10.1 2014 locked to Kitkat. It runs fast, smooth, and well on my devices. I'm very biased to KK because when LP first came out for my VZW Note 4, I updated it. Less than a day running LP and I already disliked what I call "white blindness" of all the menus... they took away the item separators so everything is just white! Also, the floating icon in Contacts gets in the way of long names forcing me to constantly scroll the list just to read the name. They also changed the color of the Notification shade pull down... it got lighter, which makes contrast with white lettering harder to see. Those are just some of the things I don't like... there are more, but it was mostly the UI that I disliked. After that less-than-a-day-run, I reverted my Note 4 back to KK. And luckily so, because with the next updates the carriers pushed out, they updated the bootloader so you couldn't downgrade anymore. A lot of people got stuck on LP whether they liked it or not! That was a few months ago, but since then, XDA members have not only unlocked the bootloader but also obtained permanent root for Verizon Note 4's. When I got my tablet, I locked it to KK right away! I will never update my devices beyond KK... but that's just me.
If you decide you want to try LP, do research first to see if your phone can be reverted back to KK. If for some reason you don't like LP, you might end up stuck on it for a long time! Lastly, you should look into blocking the automatic update so the phone doesn't just update while you're sleeping or something. And if LP has already downloaded to your phone, you might want to clear the cache partition in recovery or the phone can force the update on it's own, even if you elect not to have it update when it asks you.
Then again, LP might be nice for you. My preference is Kitkat!