You can try to modify the config file. The app seem to support more modes than the GUI does display.
Look in /Android/data/atua.anddev.uae4all2/files/conf/uaeconfig.conf
Open this file in a text editor, you can see there are three lines like this
cpu=1
chipset=2
Cpu=-4
First cpu line is the cpu type 0=68000, 1=68020
Chipset 2 is AGA
Secobd cpu lines is frequency cpu=-0 is 7mhz, -1 is 14mhz, -2 = 28mhz, -3 = 56mhz, -4 = 112mhz...
If setting this to cpu=-5 and restarting uae4all the GUI will show nothing selected in the cpu speed area.
I just tested this on my Galaxy tab4 and sysinfo does report that the speed is indeed diubled compared to the 112mhz setting.
There is also another cpu setting in that config file, mainMenu_cpuSpeed=600
I have not tried changing this yet, so this might have an imoact as well.