There's no need for what you are doing, the Vita has plenty benough resources to emulate Lion King AGA without slow-down, with 48 kHz sound, with shader, in A1200 mode, and still plenty of CPU time left.
Something must be wrong in your setup, because I get smooth gameplay 49-51 fps in Lion King AGA, using the default A1200 config. To make sure I was using defaults, I just deleted (renamed) my config using Vitashell. Then I started the emulator (v1.92), I ONLY selected "A1200" on main config, went to hard disc config, mounted my hdf files, and booted the game. I didn't change any other config.
This means in my config, shader was on, A1200 was set to 14 MHz, sound at 48 kHz, etc... all the defaults.
I have two suspicions:
a) The only difference seems to be that you are booting from hd dir, not hdf. (Apart from b), see below).
My suggestion: can you double-check using default configuration options and booting a hdf in your case (not hd dir)? See PM I sent. To see if you get full speed emulation in that case? That would mean hd dir is somehow slowing things down.
Be aware, you might have a game specific config saved. When you mount your hd dir or hdf, make sure none of your configuration options suddenly change. UAE looks for a game-specific config file when inserting new disks or HDs. You can look for those configs in the ux0:/data/uae4all2/config folder to make sure there are none. Or you can use the main menu "delete config" option to delete those conveniently.
b) You are running some kind of plugin that prevents UAE4All2 from increasing the CPU and Bus frequencies on startup. The Emulator starts with code like this:
scePowerSetArmClockFrequency(444);
scePowerSetGpuClockFrequency(222);
scePowerSetBusClockFrequency(222);
scePowerSetGpuXbarClockFrequency(222);
To increase speed when it starts.
Almost all homebrew does that. I could imagine maybe you have some plugin running that somehow overrides those commands.
So maybe try to disable all plugins apart from storagemgr. To see if a plugin is to blame.
EDIT: The more I think about it, I feel like probably b) is the problem... but we will see.
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Here's my config I used to test Lion King:
https://mega.nz/#!VIAFzIAJ!yMzA2dmzH5-pBzCdTjGdXJ8r2JD2rGSr3IHBWyrxdsQ
But as I said, it is just the default UAE starts up with, then selected "A1200", then mounted hdf.
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I could imagine you might have some plugin that "resets" the Vita clock settings to stock, even though UAE is trying to increase them. Or maybe even a plugin that messes with display (something that tries to hook into the display routine to display a menu or something).
The only plugins I have running are: storagemgr, nonpdrm, nopsmdrm, henkaku, and download_enabler
