Slowness

As soon as you enable RUST_LOG, even for just a small module that logs infrequently, the performance of your program is going to go drastically downhill.

Example: https://i.imgur.com/al4BJUn.png

+   94.04%     0.39%  rust-bitcoin-in  rust-bitcoin-indexer  [.] <serde_json::read::IoRead<R> as serde_json::read::Read<
+   93.61%     0.48%  rust-bitcoin-in  rust-bitcoin-indexer  [.] serde_json::read::next_or_eof                          
+   93.19%     0.60%  rust-bitcoin-in  rust-bitcoin-indexer  [.] std::io::read_one_byte                                 
+   92.55%     0.14%  rust-bitcoin-in  rust-bitcoin-indexer  [.] std::io::impls::<impl std::io::Read for &'a mut R>::rea
+   92.43%     1.50%  rust-bitcoin-in  rust-bitcoin-indexer  [.] <hyper::client::response::Response as std::io::Read>::r
+   91.04%     0.27%  rust-bitcoin-in  rust-bitcoin-indexer  [.] <hyper::http::h1::Http11Message as std::io::Read>::read
+   90.72%     2.96%  rust-bitcoin-in  rust-bitcoin-indexer  [.] <hyper::http::h1::HttpReader<R> as std::io::Read>::read
-   86.85%    82.85%  rust-bitcoin-in  rust-bitcoin-indexer  [.] log::__log                                             
   - 80.92% <serde_json::read::IoRead<R> as serde_json::read::Read<'de>>::parse_str                                     
      + serde_json::read::next_or_eof                                                                                   
   + 4.00% log::__log                                                                                                   
   + 1.91% jsonrpc::client::Client::send_request                                                                        
+    2.44%     0.02%  rust-bitcoin-in  rust-bitcoin-indexer  [.] rust_bitcoin_indexer::db::parse_node_block             
+    2.20%     0.00%  rust-bitcoin-in  rust-bitcoin-indexer  [.] jsonrpc::client::Client::send_request                  
+    2.18%     0.00%  rust-bitcoin-in  rust-bitcoin-indexer  [.] serde::de::impls::<impl serde::de::Deserialize<'de> for
+    2.18%     0.00%  rust-bitcoin-in  rust-bitcoin-indexer  [.] <&'a mut serde_json::de::Deserializer<R> as serde::de::
+    2.17%     0.00%  rust-bitcoin-in  rust-bitcoin-indexer  [.] serde_json::de::from_reader                            
+    2.01%     1.91%  rust-bitcoin-in  rust-bitcoin-indexer  [.] log::logger                                            
+    1.71%     0.43%  rust-bitcoin-in  rust-bitcoin-indexer  [.] <env_logger::Logger as log::Log>::log                  
+    1.59%     0.13%  rust-bitcoin-in  rust-bitcoin-indexer  [.] core::fmt::write                                       
+    1.44%     1.21%  rust-bitcoin-in  rust-bitcoin-indexer  [.] bitcoin::util::misc::he                     

As you can see, just because serde_json::read::Read has some trace! in parse_str somewhere, is completely destroying my program, for which I've enabled debugging just for my main module.

Is there anything that can be done about it?

Start by reproducing the slowdown with RUST_LOG and profiling the logging path named in the report, especially log::__log, log::logger, and env_logger::Logger::log. Compare behavior with logging disabled and with a narrowly scoped module filter; done means identifying a supported performance improvement and validating it with measured profiling results and relevant regression coverage.