diff --git a/src/widgets/accessmanagerreply_p.h b/src/widgets/accessmanagerreply_p.h
--- a/src/widgets/accessmanagerreply_p.h
+++ b/src/widgets/accessmanagerreply_p.h
@@ -96,6 +96,7 @@
 
 private:
     QByteArray m_data;
+    qint64 m_offset;
     bool m_metaDataRead;
     bool m_ignoreContentDisposition;
     bool m_emitReadyReadOnMetaDataChange;
diff --git a/src/widgets/accessmanagerreply_p.cpp b/src/widgets/accessmanagerreply_p.cpp
--- a/src/widgets/accessmanagerreply_p.cpp
+++ b/src/widgets/accessmanagerreply_p.cpp
@@ -32,6 +32,7 @@
 #include <kprotocolinfo.h>
 #include <qmimedatabase.h>
 
+#include <QtMath>
 #include <QSslConfiguration>
 
 #define QL1S(x)  QLatin1String(x)
@@ -46,6 +47,7 @@
                                        bool emitReadyReadOnMetaDataChange,
                                        QObject *parent)
     : QNetworkReply(parent),
+      m_offset(0),
       m_metaDataRead(false),
       m_ignoreContentDisposition(false),
       m_emitReadyReadOnMetaDataChange(emitReadyReadOnMetaDataChange),
@@ -84,6 +86,7 @@
                                        QObject *parent)
     : QNetworkReply(parent),
       m_data(data),
+      m_offset(0),
       m_ignoreContentDisposition(false),
       m_emitReadyReadOnMetaDataChange(false)
 {
@@ -106,7 +109,8 @@
                                        QNetworkReply::NetworkError errorCode,
                                        const QString &errorMessage,
                                        QObject *parent)
-    : QNetworkReply(parent)
+    : QNetworkReply(parent),
+      m_offset(0)
 {
     setRequest(request);
     setOpenMode(QIODevice::ReadOnly);
@@ -131,21 +135,31 @@
     }
     m_kioJob.clear();
     m_data.clear();
+    m_offset = 0;
     m_metaDataRead = false;
 }
 
 qint64 AccessManagerReply::bytesAvailable() const
 {
-    return (QNetworkReply::bytesAvailable() + m_data.length());
+    return (QNetworkReply::bytesAvailable() + m_data.length() - m_offset);
 }
 
 qint64 AccessManagerReply::readData(char *data, qint64 maxSize)
 {
-    const qint64 length = qMin(qint64(m_data.length()), maxSize);
+    const qint64 length = qMin(qint64(m_data.length() - m_offset), maxSize);
+
+    if (length <= 0) {
+        return 0;
+    }
 
-    if (length) {
-        memcpy(data, m_data.constData(), length);
-        m_data.remove(0, length);
+    memcpy(data, m_data.constData() + m_offset, length);
+    m_offset += length;
+    // Remove the stale data before m_offset only if it grows to half
+    // of the total size, to avoid calling the expensive .remove function.
+    // If there's no data left, it's a trivial remove.
+    if (m_data.length() == m_offset || m_offset * 2 >= m_data.length()) {
+        m_data.remove(0, m_offset);
+        m_offset = 0;
     }
 
     return length;
@@ -395,10 +409,32 @@
 void AccessManagerReply::slotData(KIO::Job *kioJob, const QByteArray &data)
 {
     Q_UNUSED(kioJob);
-    m_data += data;
-    if (!data.isEmpty()) {
-        emit readyRead();
+    if (data.isEmpty()) {
+        return;
     }
+
+    qint64 newSizeWithOffset = m_data.size() + data.size();
+    if (newSizeWithOffset <= m_data.capacity()) {
+        // Already enough space
+    } else if (newSizeWithOffset - m_offset <= m_data.capacity()) {
+        // We get enough space with ::remove.
+        m_data.remove(0, m_offset);
+        m_offset = 0;
+    } else {
+        // We have to resize the array, which implies an expensive memmove.
+        // Do it ourselves to save m_offset bytes.
+        QByteArray newData;
+        // Leave some free space to avoid that every slotData call results in
+        // a reallocation. qNextPowerOfTwo is what QByteArray does internally.
+        newData.reserve(qNextPowerOfTwo(newSizeWithOffset - m_offset));
+        newData.append(m_data.constData() + m_offset, m_data.size() - m_offset);
+        m_data = newData;
+        m_offset = 0;
+    }
+
+    m_data += data;
+
+    emit readyRead();
 }
 
 void AccessManagerReply::slotMimeType(KIO::Job *kioJob, const QString &mimeType)