Why adding ipad (8th generation) to my macbook pro (2021, M1 Pro) as a second monitor reduces the internet connection speed? Without iPad I get 100Mbps, with iPad it's around 20 Mbps and very unstable

There are many potential factors that can affect the measured throughput. As such, without very specific network diagnostics, it is impossible to provide a definitive root-cause with available information.

Presumably your iPad and Mac are using Sidecar - are interconnected via WiFi alone - and to the internet over the same WiFi network.

WiFi is a contended medium. In simplistic terms, all devices are sharing the same available WiFi bandwidth and available internet connection - and this reduces potential network throughput for each device. The quoted network throughput for any device is a peak value - not sustained.