diff --git a/include/wx/private/webrequest.h b/include/wx/private/webrequest.h
index 0812304fac..497bbfc170 100644
--- a/include/wx/private/webrequest.h
+++ b/include/wx/private/webrequest.h
@@ -187,6 +187,10 @@ protected:
 
     void ReportDataReceived(size_t sizeReceived);
 
+    // This function can optionally be called to preallocate the read buffer,
+    // if the total amount of data to be downloaded is known in advance.
+    void PreAllocBuffer(size_t sizeNeeded);
+
 private:
     // Called by wxWebRequestImpl only.
     friend class wxWebRequestImpl;
diff --git a/include/wx/private/webrequest_curl.h b/include/wx/private/webrequest_curl.h
index d0e2f8a5d7..25b90a80ba 100644
--- a/include/wx/private/webrequest_curl.h
+++ b/include/wx/private/webrequest_curl.h
@@ -114,11 +114,12 @@ public:
     // Methods called from libcurl callbacks
     size_t CURLOnWrite(void *buffer, size_t size);
     size_t CURLOnHeader(const char* buffer, size_t size);
-    int CURLOnProgress();
+    int CURLOnProgress(curl_off_t);
 
 private:
     wxWebRequestHeaderMap m_headers;
     wxString m_statusText;
+    wxFileOffset m_knownDownloadSize;
 
     CURL* GetHandle() const
     { return static_cast<wxWebRequestCURL&>(m_request).GetHandle(); }
diff --git a/src/common/webrequest.cpp b/src/common/webrequest.cpp
index 351a4b76ae..70dbf7232b 100644
--- a/src/common/webrequest.cpp
+++ b/src/common/webrequest.cpp
@@ -688,6 +688,11 @@ void* wxWebResponseImpl::GetDataBuffer(size_t sizeNeeded)
     return m_readBuffer.GetAppendBuf(sizeNeeded);
 }
 
+void wxWebResponseImpl::PreAllocBuffer(size_t sizeNeeded)
+{
+    m_readBuffer.SetBufSize(sizeNeeded);
+}
+
 void wxWebResponseImpl::ReportDataReceived(size_t sizeReceived)
 {
     m_readBuffer.UngetAppendBuf(sizeReceived);
diff --git a/src/common/webrequest_curl.cpp b/src/common/webrequest_curl.cpp
index d247e844af..f8d2aac47d 100644
--- a/src/common/webrequest_curl.cpp
+++ b/src/common/webrequest_curl.cpp
@@ -67,7 +67,7 @@ static size_t wxCURLHeader(char *buffer, size_t size, size_t nitems, void *userd
     return static_cast<wxWebResponseCURL*>(userdata)->CURLOnHeader(buffer, size * nitems);
 }
 
-int wxCURLXferInfo(void* clientp, curl_off_t WXUNUSED(dltotal),
+int wxCURLXferInfo(void* clientp, curl_off_t dltotal,
                    curl_off_t WXUNUSED(dlnow),
                    curl_off_t WXUNUSED(ultotal),
                    curl_off_t WXUNUSED(ulnow))
@@ -75,7 +75,7 @@ int wxCURLXferInfo(void* clientp, curl_off_t WXUNUSED(dltotal),
     wxCHECK_MSG( clientp, 0, "invalid curl progress callback data" );
 
     wxWebResponseCURL* response = reinterpret_cast<wxWebResponseCURL*>(clientp);
-    return response->CURLOnProgress();
+    return response->CURLOnProgress(dltotal);
 }
 
 int wxCURLProgress(void* clientp, double dltotal, double dlnow, double ultotal,
@@ -90,6 +90,8 @@ int wxCURLProgress(void* clientp, double dltotal, double dlnow, double ultotal,
 wxWebResponseCURL::wxWebResponseCURL(wxWebRequestCURL& request) :
     wxWebResponseImpl(request)
 {
+    m_knownDownloadSize = 0;
+
     curl_easy_setopt(GetHandle(), CURLOPT_WRITEDATA, static_cast<void*>(this));
     curl_easy_setopt(GetHandle(), CURLOPT_HEADERDATA, static_cast<void*>(this));
 
@@ -152,8 +154,17 @@ size_t wxWebResponseCURL::CURLOnHeader(const char * buffer, size_t size)
     return size;
 }
 
-int wxWebResponseCURL::CURLOnProgress()
+int wxWebResponseCURL::CURLOnProgress(curl_off_t total)
 {
+    if ( m_knownDownloadSize != total )
+    {
+        if ( m_request.GetStorage() == wxWebRequest::Storage_Memory )
+        {
+            PreAllocBuffer(static_cast<size_t>(total));
+        }
+        m_knownDownloadSize = total;
+    }
+
     return 0;
 }