I'm not really familiar with JXDs but see this :http://boards.dingoonity.org/jxd-devices/cwm-recovery-for-jxd-s7800b/
Once you got your recovery flashed, it should be as simple as putting your zipped custom rom on your sd card (or sideloading with ADB or mount it as USB storage), format /system and /data, mount /system and /data and install the custom rom.
(Prove me if i'm wrong but i did this with lots of device and jxds seem to be no different)
As for performance, here are some of my tips.
I'm assuming you want performance at the expense of memory usage and battery.
If you're rooted, install Kernel Adiutor, run it and grant it permission.
Go to CPU and set the "CPU Governor" to "Performance". Check apply on Boot.
Go to GPU. If you can change the clock, do it and set it to the maximum. Check apply on Boot.
Go to I/O Scheduler and set "Read-Ahead" to 4096Kb. Do it for your sdcard as well.
Changing the Scheduler is a more complicated task and depends on how you use your device.
If you have little to no background applications and want to break benchmark, definitively use noop.
If not, Deadline should suffice. Don't forget to check apply on Boot.
Then go to "Entropy" and set both Read and Write to 4096. Again, Check apply on Boot.
In Tools->Build prop Editor, look for hwui.disable_vsync.
If it exists, change its value to true. If not, add it and set its value to true.
There's so much you can do but for now, it should help you boosting your device a bit.