Installed lineage-19.1-20221111-UNOFFICIAL-gts28wifi.zip and is seems to be fine but I have found one bug. When you turn on the internet speed counter to be visible on the screen it always shows 0 for up and down.
The above is not a bug. In FAQ 17, LineageOS talks about legacy devices which the S2 tablets are. Since our 3.x kernel will never support eBPF, it won't count packets being sent/received which is why it always show 0 bytes.
If you use 18.1, that functionality should work, but I can't be sure since I never use it and my T710/T810 are doing some other long term tests at the moment and I can't reboot and flash to confirm.
Second, thing which is most important for me is how to limit CPU performance in order to save battery?
T719 if warm when I use it, T710 becomes hot I mean really hot that I can't even keep my finger on the screen in the right middle to the top area.
Turning on extreme battery save doesn't change too much.
If you're just using the T710 as an e-reader and the screen is super hot, then there's something wrong. Did you do a clean install + FORMAT DATA in TWRP?
You can try 18.1 to see if it's any better. In 19.1, I used a new power profile/code vs 18.1.
I've never had hot touch problems in 18.1 or 19.1 I know @lpedia
uses his T710 as an e-reader and maybe he can offer some observations?
Is is possible to add in setup this slider as on LOS16?
If it's not in LineageOS 19.1, then it's not included. I can't add features that have been deprecated. That's way too much work and likely to break things when porting a feature 3 generations old.
Which software I can use (having root) to limit CPU or change governor to powersave?
I don't need too much performance, for me it is mainly ebook reader.
You can try a variety of kernel managers to change the governor, but I haven't tested anything other than interactive. There could be bugs in the other governors including no deep sleep, random reboots, etc. I won't be troubleshooting any of that kernel code.
If battery life is important to you, I suggest:
1. Since this is your backup device, experiment without using gapps. Aurora Store will download the exact same apps from playstore and many apps will work without gapps.
2. Enable airplane mode and only have wifi on, gps/bluetooth off.
3. Look for ad-free open source apps.
4. Enable battery saver.
Just remember that since you bought the T710 used, there's no guarantee that the battery is in good shape. It could be degraded and not have much capacity left.