Yes, more demanding GBA games are not great, don't expect Doom or Crash Nitro to run at enjoyable speeds. Having custom firmware available would help, but not much, JZ4725B powered devices have low performance.
I wouldn't recommend the G100, unless you can get it for very cheap (got mine for $15 total, as I've received a partial refund for the battery, which holds the charge for about 15 minutes), you only want to play some less demanding GBA games (GB and GBC are bad on the original firmware, as you have to use Goomba to convert the roms), and you really like the shell.
With custom firmware, the device would run a lot of gen4 console games relatively fine, just like the RetroMini, and could be recommended to people on very tight budgets who also want the GBA SP shell. And even that's questionable, because for just a few dollars more, you can get the PowKiddy V90, which also has a GBA SP format shell, and runs everything up to PS1 without problems.
Maybe try asking Jutleys
, seems like he somehow made OpenDingux happen on the RG99, so he could possibly do the same on the G100?