You guys are right in saying that not every layout/theme is going to work well on the Pi. You are also correct to say using the network for anything other than roms will result in lag when browsing in AM. There are a few things you can do to make the best of the Pi hardware.
1. use unzipped Attract-Mode layouts. They will work zipped but they do take a performance hit.
2. use the best SD card you can get. This makes a huge difference especially if you can safely overclock it.
3. use a 64 GB SD card or smaller. Once you step past 64 GB you won't get the same performance and be able to overclock the SD card.
4. use mmal hardware decoding (I know most are already doing this)
5. run your Pi without a SD card and use a good USB flash or hard drive
https://www.raspberrypi.org/blog/pi-3-booting-part-i-usb-mass-storage-boot/6. overclock everything. Make a backup of your SD card and then push your Pi to the max!
use at your own risk. If you don't have a great high speed SD card don't use the SD overclock line.
/boot/config.txt partial for Pi 3 (not Pi 1, 2 or zero)
#Overclock Settings
#CPU Based
arm_freq=1350
over_voltage=6
temp_limit=80
core_freq=500
#GPU Based
h264_freq=333
avoid_pwm_pll=1
gpu_mem=450
v3d_freq=500
#Ram Overclock
sdram_freq=588
sdram_schmoo=0x02000020
over_voltage_sdram_p=6
over_voltage_sdram_i=4
over_voltage_sdram_c=4
#SD Card Overclock
dtparam=sd_overclock=100
@verion - FLAT BLUE runs ok on a Pi if it's setup right. I haven't figured out why it doesn't work unzipped on the Pi but that would help too.