https://github.com/hanabi1224/Programming-Language-Benchmarks/blob/main/bench/algorithm/edigits/1.java
is significantly faster than
https://github.com/hanabi1224/Programming-Language-Benchmarks/blob/main/bench/algorithm/edigits/1.cs

Assumption: the difference is in the biginteger implementation. Profile the C# version and investigate where the time is spent in the BigInteger implementation. Propose improvements to BigInteger, try each in turn and keep any that improve performance of this scenario.