diff --git a/lib/Interpreter/BackendPasses.cpp b/lib/Interpreter/BackendPasses.cpp
index db37819a..dcf71dac 100644
--- a/lib/Interpreter/BackendPasses.cpp
+++ b/lib/Interpreter/BackendPasses.cpp
@@ -104,13 +104,18 @@ void BackendPasses::CreatePasses(llvm::Module& M, int OptLevel)
     Inlining = CodeGenOptions::NormalInlining;
 #endif
 
-  // Handle disabling of LLVM optimization, where we want to preserve the
-  // internal module before any optimization.
-  if (m_CGOpts.DisableLLVMPasses) {
-    OptLevel = 0;
-    // Always keep at least ForceInline - NoInlining is deadly for libc++.
-    // Inlining = CGOpts.NoInlining;
-  }
+// Revert https://github.com/root-project/root/commit/548eca7 bringing back
+// optimisation level to O0 given the very long time needed to JIT simple
+// pieces of code.
+//   // Handle disabling of LLVM optimization, where we want to preserve the
+//   // internal module before any optimization.
+//   if (m_CGOpts.DisableLLVMPasses) {
+//     OptLevel = 0;
+//     // Always keep at least ForceInline - NoInlining is deadly for libc++.
+//     // Inlining = CGOpts.NoInlining;
+//   }
+
+  OptLevel = 0;
 
   llvm::PassManagerBuilder PMBuilder;
   PMBuilder.OptLevel = OptLevel;
diff --git a/lib/Interpreter/CIFactory.cpp b/lib/Interpreter/CIFactory.cpp
index 052895b6..9df199e5 100644
--- a/lib/Interpreter/CIFactory.cpp
+++ b/lib/Interpreter/CIFactory.cpp
@@ -1132,14 +1132,17 @@ static void stringifyPreprocSetting(PreprocessorOptions& PPOpts,
     CGOpts.EmitCodeView = 1;
     CGOpts.CXXCtorDtorAliases = 1;
 #endif
-    // Reduce amount of emitted symbols by optimizing more.
-    CGOpts.OptimizationLevel = 2;
-    // Taken from a -O2 run of clang:
-    CGOpts.DiscardValueNames = 1;
-    CGOpts.OmitLeafFramePointer = 1;
-    CGOpts.UnrollLoops = 1;
-    CGOpts.VectorizeLoop = 1;
-    CGOpts.VectorizeSLP = 1;
+// Revert https://github.com/root-project/root/commit/548eca7 bringing back
+// optimisation level to O0 given the very long time needed to JIT simple
+// pieces of code.
+//     // Reduce amount of emitted symbols by optimizing more.
+//     CGOpts.OptimizationLevel = 2;
+//     // Taken from a -O2 run of clang:
+//     CGOpts.DiscardValueNames = 1;
+//     CGOpts.OmitLeafFramePointer = 1;
+//     CGOpts.UnrollLoops = 1;
+//     CGOpts.VectorizeLoop = 1;
+//     CGOpts.VectorizeSLP = 1;
 
     // CGOpts.setDebugInfo(clang::CodeGenOptions::FullDebugInfo);
     // CGOpts.EmitDeclMetadata = 1; // For unloading, for later