Speed up builds by using thin LTO and default codegen-units for release builds (#5879)

Cargo.toml

@@ -744,8 +744,12 @@ version = "1.8.2"
 version = "3.1.0"
 
 [profile.release]
-codegen-units = 1 # Reduce the number of codegen units to increase optimizations
-lto = true        # Enable fat LTO
+# This is not the most performant build we could possibly do, but other
+# optimisations like `codegen-units = 1` and fat LTO are quite expensive on
+# compilation time, with relatively modest perf advantages over just thin LTO
+# (we saw 3-5% perf difference) and we'd rather optimize for compilation time
+# at the moment.
+lto = "thin"
 
 # A few profile opt-level tweaks to make the test suite run faster
 [profile.dev.package]