[LG Optimus L90] Floating Point:
I recommend to remove the software after flashing in TWRP, before doing the first boot, just mount /system and open ADB. If you do this after install, there is probably a lot of .dex files and app data remaining taking precious storage.

For example, if you update Youtube, Gmail and other pre-installed apps, after update they won't be removed, instead the new version will be renamed to something like com.google.youtube-2 and the previous version will remain in /system, you can notice this if you go to your app list in setting and try to uninstall, android will say that the version will be reverted to the stock one that shipped in the stock rom.

Here's a list of stuff and apps I removed before booting:

rm /system/usbautorun.iso

rm -r /system/app/Books

rm -r /system/app/ChromeWithBrowser

rm -r /system/app/Drive

rm -r /system/app/Gmail2

rm -r /system/app/GoogleTTS

rm -r /system/app/Hangouts

rm -r /system/app/LGSearchWidgetProvider

rm -r /system/app/Maps

rm -r /system/app/Music2

rm -r /system/app/Newsstand

rm -r /system/app/PlayGames

rm -r /system/app/PlusOne

rm -r /system/app/Street

rm -r /system/app/talkback

rm -r /system/app/Videos

rm -r /system/app/YouTube

rm -r /system/apps/bootup/LGBoxnet

rm -r /system/apps/bootup/LGFlashlightWidget

rm -r /system/apps/bootup/LGSmartWorld

rm -r /system/priv-app/LGBrowser

rm -r /system/priv-app/LGDictionary

rm -r /system/priv-app/LGEmail

rm -r /system/priv-app/LGFileManager

rm -r /system/priv-app/LGMemo

rm -r /system/priv-app/LGQTranslator

rm -r /system/priv-app/Velvet

rm -r /system/vendor/carrier/system/LGRemoteCall

rm -r /system/vendor/carrier/system/rspermlge

rm -r /system/vendor/overlay/com.android.browser

rm -r /system/vendor/overlay/com.lge.email

rm -r /system/vendor/overlay/com.lge.filemanager

rm -r /system/vendor/overlay/com.lge.lgworld

Of course, this is MY case, I removed applications that I thought not to be necessary to me. Apps such as Gmail, Youtube and etc I installed the most recent version from play store. Note that some apps cannot be removed or your cannot go past the setup wizard (the first screen after the first boot) e.g.: webview.

Don't try to install Xposed Framework, you will end with a lot of FC that will render the system unusable, I tried and didn't worked, but I had a TWRP backup and was just a matter of reverting everything.

Also, disable automatic system update. If you leave this checked in the settings and your phone updates, you will end in a soft brick.

To go to recovery you have 2 ways:

1 - Install Android Terminal Emulator, type su, then reboot recovery.

2 - Turn off your phone. Hold PWR + VOL DOWN, wait for LG logo show up, immediately release PWR button and hold again (don't release VOL DOWN). Don't worry, since you have TWRP recovery installed instead of LG recovery, your data won't be reseted, instead it will boot into TWRP.

I personally don't like the this second method because it's a good way to worn out your pwr and vol buttons, my previous phone was a LG Optimus 2x and the pwr/wake button became almost unusable after some time, I need to press very hard to work, use this second method only if you cannot boot into recovery from the OS.

To avoid always typing those command in Android Terminal Emulator, you can create a shortcut widget, just type reboot recovery in the argument, name the shortcut and it's done, everytime you tap the shortcut it will automatically open the terminal and reboot the phone. Don't forget to configure the app. to open always with su or the shortcut won't work (reboot command needs superuser access).