diff --git a/.github/workflows/fuzzers.yml b/.github/workflows/fuzzers.yml
index 69309c55c..8f9eba87a 100644
--- a/.github/workflows/fuzzers.yml
+++ b/.github/workflows/fuzzers.yml
@@ -19,6 +19,9 @@ jobs:
       implementations: haswell westmere fallback
       UBSAN_OPTIONS: halt_on_error=1
       MAXLEN: -max_len=4000
+      CLANGVERSION: 11
+      # which optimization level to use for the sanitizer build (see build_fuzzer.variants.sh)
+      OPTLEVEL: -O3
 
     steps:
     - name: Install packages necessary for building
@@ -27,7 +30,7 @@ jobs:
         sudo apt-get install --quiet ninja-build valgrind zip unzip
         wget https://apt.llvm.org/llvm.sh
         chmod +x llvm.sh
-        sudo ./llvm.sh 10
+        sudo ./llvm.sh $CLANGVERSION
 
     - uses: actions/checkout@v1
 
@@ -51,7 +54,7 @@ jobs:
         clang++ --version
 
     - name: Build all the variants
-      run: fuzz/build_fuzzer_variants.sh
+      run: CLANGSUFFIX=-$CLANGVERSION fuzz/build_fuzzer_variants.sh
 
     - name: Explore fast (release build, default implementation)
       run: |
@@ -71,7 +74,7 @@ jobs:
           others=$(find out -type d -not -name $fuzzer -not -name out -not -name cmin)
           for implementation in $implementations; do
             export SIMDJSON_FORCE_IMPLEMENTATION=$implementation
-            build-sanitizers/fuzz/fuzz_$fuzzer out/$fuzzer $others seedcorpus -max_total_time=20 $MAXLEN
+            build-sanitizers$OPTLEVEL/fuzz/fuzz_$fuzzer out/$fuzzer $others seedcorpus -max_total_time=20 $MAXLEN
           done
           echo now have $(ls out/$fuzzer |wc -l) files in corpus        
         done
@@ -82,7 +85,7 @@ jobs:
         for fuzzer in $implfuzzers; do
           # get input from everyone else (corpus cross pollination)
           others=$(find out -type d -not -name $fuzzer -not -name out -not -name cmin)
-          build-sanitizers/fuzz/fuzz_$fuzzer out/$fuzzer $others seedcorpus -max_total_time=20 $MAXLEN
+          build-sanitizers$OPTLEVEL/fuzz/fuzz_$fuzzer out/$fuzzer $others seedcorpus -max_total_time=20 $MAXLEN
           echo now have $(ls out/$fuzzer |wc -l) files in corpus
         done
 
diff --git a/fuzz/build_fuzzer_variants.sh b/fuzz/build_fuzzer_variants.sh
index 78c51fdc8..322b406c1 100755
--- a/fuzz/build_fuzzer_variants.sh
+++ b/fuzz/build_fuzzer_variants.sh
@@ -4,17 +4,27 @@
 # - different sanitizers
 # - different build options
 # - reproduce build, for running through valgrind
+#
+# Set environment variable CLANGSUFFIX to select clang version (example: "-11")
 
 # fail on error
-set -eu
+set -e
 
 unset CXX CC CFLAGS CXXFLAGS LDFLAGS
 
 me=$(basename $0)
 
+
+if [ -z $CLANGSUFFIX ] ; then
+    # the default clang version is set low enough to be found on current Debian stable (Buster)
+    CLANGSUFFIX=-8
+fi
+
+# detect unset variables
+set -u
+
 # common options
-CLANGVER=-9
-COMMON="-GNinja -DCMAKE_CXX_COMPILER=clang++$CLANGVER -DCMAKE_C_COMPILER=clang$CLANGVER -DSIMDJSON_BUILD_STATIC=Off -DENABLE_FUZZING=On -DSIMDJSON_COMPETITION=OFF -DSIMDJSON_GOOGLE_BENCHMARKS=OFF -DSIMDJSON_GIT=Off"
+COMMON="-GNinja -DCMAKE_CXX_COMPILER=clang++$CLANGSUFFIX -DCMAKE_C_COMPILER=clang$CLANGSUFFIX -DSIMDJSON_BUILD_STATIC=Off -DENABLE_FUZZING=On -DSIMDJSON_COMPETITION=OFF -DSIMDJSON_GOOGLE_BENCHMARKS=OFF -DSIMDJSON_GIT=Off"
 
 # A replay build, as plain as it gets. For use with valgrind/gdb.
 variant=replay
@@ -33,17 +43,28 @@ fi
 
 
 # A fuzzer with sanitizers. For improved capability to find bugs.
-variant=sanitizers
+# About the optimization level: (measured for fuzz_atpointer)
+# -O0 gives  4000 executions/s
+# -O1 gives 20000 executions/s
+# -O2 gives 32000 executions/s
+# -O3 gives 32000 executions/s
+# for reference, the release build (without sanitizers, but with fuzzing instrumentation)
+# gives 80000 executions/s.
+# A low level is good for debugging. A higher level gets more work done.
+# Different levels may uncover different types of bugs, see this interesting
+# thread: https://github.com/google/oss-fuzz/issues/2295#issuecomment-481493392
+# Oss-fuzz uses -O1 so it may be relevant to use something else than that,
+# to do something oss-fuzz doesn't.
+variant=sanitizers-O3
 
     if [ ! -d build-$variant ] ; then
 	
 	mkdir build-$variant
 	cd build-$variant
-	
 	cmake .. \
 	      $COMMON \
-	      -DCMAKE_CXX_FLAGS="-fsanitize=fuzzer-no-link,address,undefined -fno-sanitize-recover=undefined" \
-	      -DCMAKE_C_FLAGS="-fsanitize=fuzzer-no-link,address,undefined -fno-sanitize-recover=undefined" \
+	      -DCMAKE_CXX_FLAGS="-O3 -fsanitize=fuzzer-no-link,address,undefined -fno-sanitize-recover=undefined" \
+	      -DCMAKE_C_FLAGS="-O3 -fsanitize=fuzzer-no-link,address,undefined -fno-sanitize-recover=undefined" \
 	      -DCMAKE_BUILD_TYPE=Debug \
 	      -DSIMDJSON_FUZZ_LINKMAIN=Off \
 	      -DSIMDJSON_FUZZ_LDFLAGS="-fsanitize=fuzzer"
@@ -52,6 +73,23 @@ variant=sanitizers
 	cd ..
     fi
 
+variant=sanitizers-O0
+
+    if [ ! -d build-$variant ] ; then
+	
+	mkdir build-$variant
+	cd build-$variant
+	cmake .. \
+	      $COMMON \
+	      -DCMAKE_CXX_FLAGS="-O0 -fsanitize=fuzzer-no-link,address,undefined -fno-sanitize-recover=undefined" \
+	      -DCMAKE_C_FLAGS="-O0 -fsanitize=fuzzer-no-link,address,undefined -fno-sanitize-recover=undefined" \
+	      -DCMAKE_BUILD_TYPE=Debug \
+	      -DSIMDJSON_FUZZ_LINKMAIN=Off \
+	      -DSIMDJSON_FUZZ_LDFLAGS="-fsanitize=fuzzer"
+	
+	ninja all_fuzzers
+	cd ..
+    fi
 
 
 # A fast fuzzer, for fast exploration rather than finding bugs.
diff --git a/fuzz/quick_check.sh b/fuzz/quick_check.sh
index 0dc30c58f..fce257274 100755
--- a/fuzz/quick_check.sh
+++ b/fuzz/quick_check.sh
@@ -27,7 +27,11 @@ if [ ! -d out ] ; then
   tar xf corpus.tar && rm corpus.tar
 fi
 
-builddir=build-sanitizers
+# By default, use the debug friendly variant since this script is intended
+# for developers reproducing bugs/validating fixes locally.
+builddir=build-sanitizers-O0
+#...but feel free to use this one instead, if you want to build coverage fast.
+#builddir=build-fast
 
 if [ ! -d $builddir ] ; then
   fuzz/build_fuzzer_variants.sh