[LG L Fino ROMs, Kernels, Recoveries, & Other Devel] christpp:
I will be changing the IO scheduler for the SD card to ROW in the next update, which shows some minor speed improvements in some apps on adoptable storage. In the meantime, you can change it yourself and see if it makes things better using a kernel tuning app from the play store.

The issue of corrupted/not recognized SD card while phone is in sleep mode is caused by a soft-reboot which the phone enters for some reason. I believe this is Android related and fixed in latest updates of LineageOS (I have been running some test builds on my device for a week and I haven't faced this issue yet). A soft-reboot while the card is not unmounted correctly might cause a corruption of the card. The message you see behind the reboot dialog is coming from the Update Notification app I think, which I enabled in the last couple of builds. It should be notifying about the need for rescheduling the service after boot.

If you have lots of apps installed on adoptable storage, it might make booting slower. At startup, Android is scanning storage for all the apps installed before going into the main screen. Our SD card read speed (formatted as adoptable) shouldn't be higher than 5-10 mb/s, so it is taking a lot of time to scan the whole card. Apps that include services with broadcast receivers (ones that listen to the boot completed intent, eg. Facebook, Hangouts) should not be placed on the Adoptable storage. After repairing the f2fs on you linux box, I believe android is not starting up any services placed on the adoptable storage, until you enter the application that includes it, that is why it is starting up faster(just my thought though).

I am not familiar with any way to force the filesystem used on adopted storage. Android developers documentation reports that the filesystem type is chosen by the system based on the kernel capabilities. I will look into it.

EDIT: You can try changing the partition type yourself and see how it goes. Most probably android will refuse to load it though

So to sum things up:

What is making adoptable storage slow is the msm8610 board's SD card chip and not the SD card itself. Even with the latest and most capable card, the chip will have specific highest read/write speeds. Even if you see some increases in benchmarks, it might be coming from bad kernel calculations(the readahead cache and several other factors can give misleading results).

Slow read speeds also mean that higher capacity SD cards will make things worse. Android system will need much more time to read more MBs of data. I have 4GB adopted and my device still struggles with that.

On lower end devices, special care must be taken on what apps will be installed on the Adopted Storage. Applications that need to be started up at device boot (this includes most messenger apps, battery savers, cache cleaners, kernel tuning apps etc), widgets, launchers should not be installed on Adopted Storage.

That is all I am currently aware of, I am still trying to find ways to tune things more. We are having a budget device, with very limited capabilities and almost outdated, so chances for better results are small anyway.