When game become complex and many units are moving, client may hang or lock up.
Here are some settings you may have to look at that are known to make client faster.
1) Use Trident tileset
2) Change animation times. Less bling make client fater. I use 15/100/10 ms. Consider turning off combat animation.
3) Disable all "auto center" and "focus on awakened units" and popups.
4) Consider using a Linux distro. Freeciv appear to have less bugs on linux.
I have added some pictures showing my local settings.