diff --git a/src/libexpr/eval-cache.cc b/src/libexpr/eval-cache.cc
index ac646af8a..9e734e654 100644
--- a/src/libexpr/eval-cache.cc
+++ b/src/libexpr/eval-cache.cc
@@ -625,7 +625,8 @@ string_t AttrCursor::getStringWithContext()
         NixStringContext context;
         copyContext(v, context);
         return {v.string.s, std::move(context)};
-    } else if (v.type() == nPath)
+    }
+    else if (v.type() == nPath)
         return {v.path().to_string(), {}};
     else
         root->state.error("'%s' is not a string but %s", getAttrPathStr()).debugThrow<TypeError>();
diff --git a/src/libutil/tarfile.cc b/src/libutil/tarfile.cc
index 238d0a7a6..5060a8f24 100644
--- a/src/libutil/tarfile.cc
+++ b/src/libutil/tarfile.cc
@@ -17,7 +17,7 @@ static ssize_t callback_read(struct archive * archive, void * _self, const void
     *buffer = self->buffer.data();
 
     try {
-        return self->source->read((char *) self->buffer.data(), 4096);
+        return self->source->read((char *) self->buffer.data(), self->buffer.size());
     } catch (EndOfFile &) {
         return 0;
     } catch (std::exception & err) {
@@ -39,7 +39,7 @@ void TarArchive::check(int err, const std::string & reason)
         throw Error(reason, archive_error_string(this->archive));
 }
 
-TarArchive::TarArchive(Source & source, bool raw) : buffer(4096)
+TarArchive::TarArchive(Source & source, bool raw) : buffer(65536)
 {
     this->archive = archive_read_new();
     this->source = &source;
diff --git a/src/libutil/tarfile.hh b/src/libutil/tarfile.hh
index 24afb710a..237d18c31 100644
--- a/src/libutil/tarfile.hh
+++ b/src/libutil/tarfile.hh
@@ -24,6 +24,7 @@ struct TarArchive {
 
     ~TarArchive();
 };
+
 void unpackTarfile(Source & source, const Path & destDir);
 
 void unpackTarfile(const Path & tarFile, const Path & destDir);
diff --git a/src/nix/build.cc b/src/nix/build.cc
index 4e133e288..abf946214 100644
--- a/src/nix/build.cc
+++ b/src/nix/build.cc
@@ -27,8 +27,10 @@ nlohmann::json builtPathsWithResultToJSON(const std::vector<BuiltPathWithResult>
         std::visit([&](const auto & t) {
             auto j = t.toJSON(store);
             if (b.result) {
-                j["startTime"] = b.result->startTime;
-                j["stopTime"] = b.result->stopTime;
+                if (b.result->startTime)
+                    j["startTime"] = b.result->startTime;
+                if (b.result->stopTime)
+                    j["stopTime"] = b.result->stopTime;
                 if (b.result->cpuUser)
                     j["cpuUser"] = ((double) b.result->cpuUser->count()) / 1000000;
                 if (b.result->cpuSystem)