glBufferSubData has peaks with small buffer sizes - c++

Question

I am making a simple 2D game engine with OpenGL, and my SpriteRenderer class makes batches of sprites which use the same shader or texture. Once I sort them first by texture, then by shader, and then by depth, the uploadData() method of my SpriteRenderer class uploads the CPU generated vertex data to the GPU. For this I am using the orphaning method, which is said to be faster than a simple glBufferData call.
My problem is, when I have small numbers of sprites (<15), I have frames with huge stalls, but with numbers >20 up to 10 000 it runs smoothly. I wrapped opengl function into c++ classes. If I replace the VertexBufferObject::uploadOrphaned() method by the VertexBufferObject::upload() method, the stall goes away. Here is my vbo class:
//vbo.h
class VertexBufferObject
{
public:
VertexBufferObject();
VertexBufferObject(size_t allocSize);
~VertexBufferObject();
inline ui32 id(){ return (ui32)m_id; }
template <class T>
void addData(const T& data)
{
const byte* convertedData = reinterpret_cast<const byte*>(&data);
size_t size = sizeof(T);
for (ui32 i = 0; i < size; i++)m_data.add(convertedData[i]);
}
void initialize();
void resetData();
void upload(GLenum drawType);
void uploadOrphaned(GLenum drawType);
inline void bind() { ::glBindBuffer(GL_ARRAY_BUFFER, m_id); }
inline void unbind(){ ::glBindBuffer(GL_ARRAY_BUFFER, 0); }
private:
GLuint m_id;
//this is my custom dynamic array class, this is a hobby project
//and I enjoy reinventing the wheel :)
Array<byte> m_data;
and below is the cpp file:
VertexBufferObject::VertexBufferObject() :m_id(0)
{
}
VertexBufferObject::VertexBufferObject(size_t allocSize) : m_id(0), m_data(allocSize)
{
}
VertexBufferObject::~VertexBufferObject()
{
if (m_id != 0) ::glDeleteBuffers(1, &m_id);
}
void VertexBufferObject::initialize()
{
if (m_id == 0) ::glGenBuffers(1, &m_id);
}
void VertexBufferObject::upload(GLenum drawType)
{
glBufferData(GL_ARRAY_BUFFER, m_data.size(), &m_data[0], drawType);
}
void VertexBufferObject::uploadOrphaned(GLenum drawType)
{
glBufferData(GL_ARRAY_BUFFER, m_data.size() * 2, NULL, drawType);
glBufferSubData(GL_ARRAY_BUFFER, 0, m_data.size(), &m_data[0]);
}
void VertexBufferObject::resetData()
{
m_data.strip();
}
I am learning opengl, so my VertexBufferObject class contains what I already know. I know there are other methods to upload data, but I want to understand the reason of the current problem with this simple method. I looked around forums including here, and nobody had a similar issue that I found of. Maybe my class has some errors?