This will be the 4.4.2 Kitkat update.
http://www.pocket-lint.com/news/1278...-kitkat-update
Unless you're an experienced user (and it doesn't look like you are, or you'd know what the update was), there isn't a way to revert to 4.3. To clarify, you could root your phone and install a 4.3 ROM, but it would void your warranty. Do NOT attempt this unless you know exactly what you're doing, or you will brick your phone and it will become useless.
You may be able to speed things up by clearing the cache. You do this in the following way:
Turn your phone off completely.
Hold the Power, Menu and Volume UP buttons down simultaneously until you see "Boot into recovery" in tiny blue letters in the top left corner of your phone.
Let go of the Power button.
That should bring up the Recovery menu - let go of the other buttons.
Use the volume buttons to scroll down to Wipe Cache Partition.
Press the power button to select.
Once this has happened, select "Reboot phone" (should be the first option) and press the power button again. Your phone will reboot / restart.
You should always do this after a major system update as it helps clear out old and obsolete files. Your phone will hopefully start to run faster.
Be careful you don't press the Wipe All Data / Return To Factory Mode option - it will do exactly as the name implies and return your phone to the exact state it was in when you bought it. You'll still have the new Kitkat update, but all your apps and accounts will have been wiped and will need to be re-installed.