I experienced similar issues several months ago, and I worked with Karol and did various testing using Blokada app with VPN enabled. I get 350 Mbps download rate as a baseline. I then tested the speed with Blokada on an Android and IOS devices. With the IOS devices, I was able to get about 90-200 Mbps download with Blokada (ad block and VPN) enabled. With Android, about 50Mbps was the max that was ever achieved. The limitations on Android are based upon how the Android operating system was developed.
As @UK_Wildcats_Fans mentioned, it’s a known limitation to Android. But if you want to nail it down to see if there are any issues with the Blokada app in particular, you might want to follow the steps in this post to compare it with the official WireGuard app:
Exact speeds in each test
Blokada for Android 5.11.1
download: 51.8Mbps upload: 70.8Mbps
Blokada for Android 5.11.1 (disable all blocklists)
download: 52.8Mbps upload: 73.3Mbps
WireGuard for Android v1.0.20210506
download: 98.8Mbps upload: 121Mbps