My ADS-B RX has been running for two months and I am still looking for some improvements to increase the range, number of airplane messages decoded. The first big step was a LNA and 1090 MHz filter between the antenna and RTL-SDR receiver. On the software side, I added an automatic gain control.
Today, I replaced the RTL-SDR stick by an Airspy R2. This SDR RX has much better performance. On the downside however, it also requires much more computing power. It pushes the Raspberry Pi 3B with 1GB RAM to its limits.
Some graphs with situation before change (RTL-SDR before 8am) and after (Airspy R2 after 8:15am) can be seen in this PDF: graphs1090_ Performance Graphs
Links to my ADS-B life data can be found here: https://dl2akt.de/links/