diff --git a/freezer/freezer.go b/freezer/freezer.go
index d95c373..0c70193 100644
--- a/freezer/freezer.go
+++ b/freezer/freezer.go
@@ -11,13 +11,18 @@ import (
 )
 
 var (
+	defaultMaxUnflushedCount  = 1024
+	defaultMaxUnflushedPeriod = time.Second
+
 	_ substrate.AsyncMessageSink   = (*asyncMessageSink)(nil)
 	_ substrate.AsyncMessageSource = (*asyncMessageSource)(nil)
 )
 
 type AsyncMessageSinkConfig struct {
-	StreamStore   straw.StreamStore
-	FreezerConfig freezer.MessageSinkConfig
+	StreamStore        straw.StreamStore
+	FreezerConfig      freezer.MessageSinkConfig
+	MaxUnflushedCount  int           // maximum number of unflushed messages
+	MaxUnflushedPeriod time.Duration // maximum period before flushing
 }
 
 func NewAsyncMessageSink(config AsyncMessageSinkConfig) (substrate.AsyncMessageSink, error) {
@@ -25,36 +30,45 @@ func NewAsyncMessageSink(config AsyncMessageSinkConfig) (substrate.AsyncMessageS
 	if err != nil {
 		return nil, err
 	}
-	return &asyncMessageSink{fms}, nil
+	if config.MaxUnflushedCount <= 0 {
+		config.MaxUnflushedCount = defaultMaxUnflushedCount
+	}
+	if config.MaxUnflushedPeriod <= 0 {
+		config.MaxUnflushedPeriod = defaultMaxUnflushedPeriod
+	}
+
+	return &asyncMessageSink{
+		fms:                fms,
+		maxUnflushedCount:  config.MaxUnflushedCount,
+		maxUnflushedPeriod: config.MaxUnflushedPeriod,
+	}, nil
 }
 
 type asyncMessageSink struct {
-	fms *freezer.MessageSink
+	fms                *freezer.MessageSink
+	maxUnflushedCount  int
+	maxUnflushedPeriod time.Duration
 }
 
 func (ams *asyncMessageSink) PublishMessages(ctx context.Context, acks chan<- substrate.Message, messages <-chan substrate.Message) (rerr error) {
 	var toAck []substrate.Message
-	t := time.NewTimer(0)
-	if !t.Stop() {
-		<-t.C
-	}
+
+	t := time.NewTicker(ams.maxUnflushedPeriod)
+	defer t.Stop()
+
 	for {
 		select {
 		case <-ctx.Done():
 			return nil
 		case m := <-messages:
-			if !t.Stop() {
-				select {
-				case <-t.C:
-				default:
-				}
-			}
-			t.Reset(1000 * time.Millisecond)
 			if err := ams.fms.PutMessage(m.Data()); err != nil {
 				return err
 			}
 			toAck = append(toAck, m)
 		case <-t.C:
+			if len(toAck) == 0 {
+				continue
+			}
 			if err := ams.fms.Flush(); err != nil {
 				return err
 			}
@@ -67,19 +81,18 @@ func (ams *asyncMessageSink) PublishMessages(ctx context.Context, acks chan<- su
 			}
 			toAck = toAck[0:0]
 		}
-		if len(toAck) > 1024 {
+		if len(toAck) >= ams.maxUnflushedCount {
 			if err := ams.fms.Flush(); err != nil {
 				return err
 			}
 			for _, m := range toAck {
 				select {
 				case <-ctx.Done():
-					return ctx.Err()
+					return nil
 				case acks <- m:
 				}
 			}
 			toAck = toAck[0:0]
-			t.Stop()
 		}
 	}
 }
diff --git a/freezer/freezer_url.go b/freezer/freezer_url.go
index 2ba3e83..8a0d360 100644
--- a/freezer/freezer_url.go
+++ b/freezer/freezer_url.go
@@ -3,8 +3,10 @@ package freezer
 import (
 	"fmt"
 	"net/url"
+	"strconv"
 	"time"
 
+	"github.com/pkg/errors"
 	"github.com/uw-labs/freezer"
 	"github.com/uw-labs/straw"
 	"github.com/uw-labs/substrate"
@@ -18,10 +20,27 @@ func init() {
 	suburl.RegisterSource("freezer+s3", newFreezerSource)
 }
 
-func newFreezerSink(u *url.URL) (substrate.AsyncMessageSink, error) {
+func newFreezerSink(u *url.URL) (sink substrate.AsyncMessageSink, err error) {
 
 	q := u.Query()
 
+	var (
+		maxUnflushedCount  = defaultMaxUnflushedCount
+		maxUnflushedPeriod = defaultMaxUnflushedPeriod
+	)
+	if count := q.Get("max-unflushed-count"); count != "" {
+		maxUnflushedCount, err = strconv.Atoi(count)
+		if err != nil {
+			return nil, errors.Wrap(err, "failed to parse max-unflushed-count")
+		}
+	}
+	if period := q.Get("max-unflushed-period"); period != "" {
+		maxUnflushedPeriod, err = time.ParseDuration(period)
+		if err != nil {
+			return nil, errors.Wrap(err, "failed to parse max-unflushed-period")
+		}
+	}
+
 	cts := q.Get("compression")
 	ct := freezer.CompressionTypeNone
 	switch cts {
@@ -44,6 +63,7 @@ func newFreezerSink(u *url.URL) (substrate.AsyncMessageSink, error) {
 
 		sse := q.Get("sse")
 		switch sse {
+		case "":
 		case "aes256":
 			enc = straw.S3ServerSideEncoding(straw.ServerSideEncryptionTypeAES256)
 		default:
@@ -67,6 +87,8 @@ func newFreezerSink(u *url.URL) (substrate.AsyncMessageSink, error) {
 			Path:            u.Path,
 			CompressionType: ct,
 		},
+		MaxUnflushedCount:  maxUnflushedCount,
+		MaxUnflushedPeriod: maxUnflushedPeriod,
 	}
 	return sinker(conf)
 }
diff --git a/freezer/freezer_url_test.go b/freezer/freezer_url_test.go
index 1b28897..a855922 100644
--- a/freezer/freezer_url_test.go
+++ b/freezer/freezer_url_test.go
@@ -28,19 +28,23 @@ func TestFreezerSink(t *testing.T) {
 					CompressionType: freezer.CompressionTypeNone,
 					Path:            "/foo/1",
 				},
-				StreamStore: &straw.OsStreamStore{},
+				StreamStore:        &straw.OsStreamStore{},
+				MaxUnflushedCount:  defaultMaxUnflushedCount,
+				MaxUnflushedPeriod: defaultMaxUnflushedPeriod,
 			},
 			expectedErr: nil,
 		},
 		{
 			name:  "everything-dir",
-			input: "freezer+dir:///foo/bar2/baz/?compression=snappy",
+			input: "freezer+dir:///foo/bar2/baz/?compression=snappy&max-unflushed-count=20&max-unflushed-period=10s",
 			expected: AsyncMessageSinkConfig{
 				FreezerConfig: freezer.MessageSinkConfig{
 					CompressionType: freezer.CompressionTypeSnappy,
 					Path:            "/foo/bar2/baz/",
 				},
-				StreamStore: &straw.OsStreamStore{},
+				StreamStore:        &straw.OsStreamStore{},
+				MaxUnflushedCount:  20,
+				MaxUnflushedPeriod: time.Second * 10,
 			},
 			expectedErr: nil,
 		},