What is the best way to turn my old Android tablet into a 100% VNC client?

I have a very old Android tablet that I barely use.

It's a Zipy MID Fun (8GB) with 256MB RAM DDR2, an old Rockchip at 660MHz with a DPS 600MHz runnind Android 2.1. The wireless module is a very weak IEEE 802.11G module.

Since my other tablet broke (someone decided that it was a nice doormat), I decided to use this one as a 100% VNC client.

The problem is that this is SO SLOW it is almost unusable. Using 256 colors is the only way to use it somewhat properly.

The VNC Server is a Windows 8.1 computer, which is connected over cable.

What should I do to use this tablet as a decent VNC client?

(RDP is also fine, if it is possible with such an old tablet)