I got Lineage OS (Android 6.0.1. Nightly build from May 20 2017) running on my Shield Portable. I already had it unlocked from when I had to recover from a bad OTA update years ago, so all I really had to do was update the recovery partition. The biggest issue was setting up the USB driver to recognize my device in adb *and* fastboot mode.
I'm pretty happy so far. It seems to be faster and more stable. Retroarch is allowing me to queue downloads, which it wasn't doing this morning on stock android.
The best reason for doing this is it will allow you to set up a big SDXC card as permanent internal storage.
I haven't tried to compile a more recent version, but if I do I'll let everyone know how it goes. If you have a portable that's been too sluggish or outdated on stock, I at least recommend trying one of the builds that's floating around.
Don't be scare.
Edit: Update. The nightly build I'm using isn't super stable. Google play services are prone to cause crashes. I have heard that the last build of CyanogenMod for the Shield is more stable. I'm happy enough with where this thing is at the moment that I'm not going to try to change it. The interface runs more quickly. Most apps are more responsive. Stability is acceptable. I'm free from the storage limitations imposed by Lollipop. Sleep actually seems to significantly save battery.
Edit: Updater. I tried to use
CyanogenMod rom with
32 bit 6.0 pico GApps for improved stability, which seemed to work at first. For LineageOS and Cyanogenmod, problems seem to come up when certain apps are installed, even when those apps aren't running. On Lineage and Cyanogenmod, trying to run ePSXe crashes the system. Worse, the package manager when I try to uninstall it. I don't know why this problem is coming up with ePSXe specifically, when I can run and manage other programs without the same problem. I'm sure I could probably use adb to get terminal output, etc. but I'm not sure it's worth it.
At this point, I'm not really sure what I'm going to do. I might go back to stock. I might switch to RetroArch instead of ePSXe. I might try running without google play services at all, and just side-load everything.