diff --git a/docs/storage.md b/docs/storage.md
index 989767feb46..8b62d290670 100644
--- a/docs/storage.md
+++ b/docs/storage.md
@@ -92,7 +92,7 @@ config:
   trace:
     enable: false
   list_objects_version: ""
-  part_size: 134217728
+  part_size: 67108864
   sse_config:
     type: ""
     kms_key_id: ""
diff --git a/go.mod b/go.mod
index e3ce8199d3f..926e451a468 100644
--- a/go.mod
+++ b/go.mod
@@ -76,6 +76,9 @@ replace (
 	github.com/bradfitz/gomemcache => github.com/themihai/gomemcache v0.0.0-20180902122335-24332e2d58ab
 	// Update to v1.1.1 to make sure windows CI pass.
 	github.com/elastic/go-sysinfo => github.com/elastic/go-sysinfo v1.1.1
+
+	// TODO: Remove this: https://github.com/thanos-io/thanos/issues/3967.
+	github.com/minio/minio-go/v7 => github.com/bwplotka/minio-go/v7 v7.0.11-0.20210324165441-f9927e5255a6
 	// Make sure Prometheus version is pinned as Prometheus semver does not include Go APIs.
 	github.com/prometheus/prometheus => github.com/prometheus/prometheus v1.8.2-0.20210215121130-6f488061dfb4
 	github.com/sercand/kuberesolver => github.com/sercand/kuberesolver v2.4.0+incompatible
diff --git a/go.sum b/go.sum
index f530ac84c64..bb70670be0b 100644
--- a/go.sum
+++ b/go.sum
@@ -195,6 +195,8 @@ github.com/blang/semver v3.5.0+incompatible/go.mod h1:kRBLl5iJ+tD4TcOOxsy/0fnweb
 github.com/bmizerany/assert v0.0.0-20160611221934-b7ed37b82869/go.mod h1:Ekp36dRnpXw/yCqJaO+ZrUyxD+3VXMFFr56k5XYrpB4=
 github.com/bmizerany/pat v0.0.0-20170815010413-6226ea591a40/go.mod h1:8rLXio+WjiTceGBHIoTvn60HIbs7Hm7bcHjyrSqYB9c=
 github.com/boltdb/bolt v1.3.1/go.mod h1:clJnj/oiGkjum5o1McbSZDSLxVThjynRyGBgiAx27Ps=
+github.com/bwplotka/minio-go/v7 v7.0.11-0.20210324165441-f9927e5255a6 h1:h9SZ0jmAKjtrZF6iZ77/jdXdHr+Usn29itI669SVRp4=
+github.com/bwplotka/minio-go/v7 v7.0.11-0.20210324165441-f9927e5255a6/go.mod h1:td4gW1ldOsj1PbSNS+WYK43j+P1XVhX/8W8awaYlBFo=
 github.com/c-bata/go-prompt v0.2.2/go.mod h1:VzqtzE2ksDBcdln8G7mk2RX9QyGjH+OVqOCSiVIqS34=
 github.com/casbin/casbin/v2 v2.1.2/go.mod h1:YcPU1XXisHhLzuxH9coDNf2FbKpjGlbCg3n9yuLkIJQ=
 github.com/cenkalti/backoff v0.0.0-20181003080854-62661b46c409/go.mod h1:90ReRw6GdpyfrHakVjL/QHaoyV4aDUVVkXQJJJ3NXXM=
@@ -900,10 +902,6 @@ github.com/minio/md5-simd v1.1.0 h1:QPfiOqlZH+Cj9teu0t9b1nTBfPbyTl16Of5MeuShdK4=
 github.com/minio/md5-simd v1.1.0/go.mod h1:XpBqgZULrMYD3R+M28PcmP0CkI7PEMzB3U77ZrKZ0Gw=
 github.com/minio/minio-go/v6 v6.0.44/go.mod h1:qD0lajrGW49lKZLtXKtCB4X/qkMf0a5tBvN2PaZg7Gg=
 github.com/minio/minio-go/v6 v6.0.56/go.mod h1:KQMM+/44DSlSGSQWSfRrAZ12FVMmpWNuX37i2AX0jfI=
-github.com/minio/minio-go/v7 v7.0.2 h1:P/7wFd4KrRBHVo7AKdcqO+9ReoS+XpMjfRFoE5quH0E=
-github.com/minio/minio-go/v7 v7.0.2/go.mod h1:dJ80Mv2HeGkYLH1sqS/ksz07ON6csH3S6JUMSQ2zAns=
-github.com/minio/minio-go/v7 v7.0.10 h1:1oUKe4EOPUEhw2qnPQaPsJ0lmVTYLFu03SiItauXs94=
-github.com/minio/minio-go/v7 v7.0.10/go.mod h1:td4gW1ldOsj1PbSNS+WYK43j+P1XVhX/8W8awaYlBFo=
 github.com/minio/sha256-simd v0.1.1 h1:5QHSlgo3nt5yKOJrC7W8w7X+NFl8cMPZm96iu8kKUJU=
 github.com/minio/sha256-simd v0.1.1/go.mod h1:B5e1o+1/KgNmWrSQK08Y6Z1Vb5pwIktudl0J58iy0KM=
 github.com/mitchellh/cli v1.0.0/go.mod h1:hNIlj7HEI86fIcpObd7a0FcrxTWetlwJDGcceTlRvqc=
diff --git a/pkg/objstore/s3/s3.go b/pkg/objstore/s3/s3.go
index 8814412e898..6a490fd5eb5 100644
--- a/pkg/objstore/s3/s3.go
+++ b/pkg/objstore/s3/s3.go
@@ -66,9 +66,7 @@ var DefaultConfig = Config{
 		MaxIdleConnsPerHost:   100,
 		MaxConnsPerHost:       0,
 	},
-	// Minimum file size after which an HTTP multipart request should be used to upload objects to storage.
-	// Set to 128 MiB as in the minio client.
-	PartSize: 1024 * 1024 * 128,
+	PartSize: 1024 * 1024 * 64, // 64MB.
 }
 
 // Config stores the configuration for s3 bucket.
@@ -85,6 +83,7 @@ type Config struct {
 	TraceConfig        TraceConfig       `yaml:"trace"`
 	ListObjectsVersion string            `yaml:"list_objects_version"`
 	// PartSize used for multipart upload. Only used if uploaded object size is known and larger than configured PartSize.
+	// NOTE we need to make sure this number does not produce more parts than 10 000.
 	PartSize  uint64    `yaml:"part_size"`
 	SSEConfig SSEConfig `yaml:"sse_config"`
 }
@@ -449,7 +448,6 @@ func (b *Bucket) Upload(ctx context.Context, name string, r io.Reader) error {
 		size = -1
 	}
 
-	// partSize cannot be larger than object size.
 	partSize := b.partSize
 	if size < int64(partSize) {
 		partSize = 0
diff --git a/pkg/objstore/s3/s3_e2e_test.go b/pkg/objstore/s3/s3_e2e_test.go
new file mode 100644
index 00000000000..e837b9baa58
--- /dev/null
+++ b/pkg/objstore/s3/s3_e2e_test.go
@@ -0,0 +1,55 @@
+// Copyright (c) The Thanos Authors.
+// Licensed under the Apache License 2.0.
+
+package s3_test
+
+import (
+	"bytes"
+	"context"
+	"strings"
+	"testing"
+
+	"github.com/cortexproject/cortex/integration/e2e"
+	e2edb "github.com/cortexproject/cortex/integration/e2e/db"
+	"github.com/go-kit/kit/log"
+	"github.com/thanos-io/thanos/pkg/objstore/s3"
+	"github.com/thanos-io/thanos/test/e2e/e2ethanos"
+
+	"github.com/thanos-io/thanos/pkg/testutil"
+)
+
+// Regression benchmark for https://github.com/thanos-io/thanos/issues/3917.
+func BenchmarkUpload(b *testing.B) {
+	b.ReportAllocs()
+	ctx := context.Background()
+
+	s, err := e2e.NewScenario("e2e_bench_mino_client")
+	testutil.Ok(b, err)
+	b.Cleanup(e2ethanos.CleanScenario(b, s))
+
+	const bucket = "test"
+	m := e2edb.NewMinio(8080, bucket)
+	testutil.Ok(b, s.StartAndWaitReady(m))
+
+	bkt, err := s3.NewBucketWithConfig(log.NewNopLogger(), s3.Config{
+		Bucket:    bucket,
+		AccessKey: e2edb.MinioAccessKey,
+		SecretKey: e2edb.MinioSecretKey,
+		Endpoint:  m.HTTPEndpoint(),
+		Insecure:  true,
+	}, "test-feed")
+	testutil.Ok(b, err)
+
+	buf := bytes.Buffer{}
+	buf.Grow(1028 * 1028 * 100) // 100MB.
+	word := "abcdefghij"
+	for i := 0; i < buf.Cap()/len(word); i++ {
+		_, _ = buf.WriteString(word)
+	}
+	str := buf.String()
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		testutil.Ok(b, bkt.Upload(ctx, "test", strings.NewReader(str)))
+	}
+}
diff --git a/test/e2e/e2ethanos/helpers.go b/test/e2e/e2ethanos/helpers.go
index 714bdbb468e..0bcd21c9128 100644
--- a/test/e2e/e2ethanos/helpers.go
+++ b/test/e2e/e2ethanos/helpers.go
@@ -16,7 +16,7 @@ import (
 	"github.com/thanos-io/thanos/pkg/testutil"
 )
 
-func CleanScenario(t *testing.T, s *e2e.Scenario) func() {
+func CleanScenario(t testing.TB, s *e2e.Scenario) func() {
 	return func() {
 		// Make sure Clean can properly delete everything.
 		testutil.Ok(t, exec.Command("chmod", "-R", "777", s.SharedDir()).Run())