qt freezes for 10+ seconds when opening context menu over c++ object

i came across a bottleneck when working on msgpack_cpp adapters for custom types: msgpack_cpp allows the creation of adaptors for custom types by template specialization of a `convert<>` struct, something like:

#include <msgpack.hpp>
namespace msgpack {
MSGPACK_API_VERSION_NAMESPACE(MSGPACK_DEFAULT_API_NS) {
namespace adaptor {

struct my_type
{};

template<>
struct convert<my_type>
{
    msgpack::object const& operator()(msgpack::object const& object, my_type& value) const;
};

}

when opening the context menu (right-click) on `operator()`, the context menu opens immediately, but qtc freezes for 10+ seconds (in my real-world codebase). most of the cpu time seems to be spent inside `CPlusPlus::ClassOrNamespace::lookupType_helper`.

msgpack_cpp brings a lot of built-in adaptors (specialisations of convert<>). so i wonder if this will cause any performance bottleneck, which can be worked around in one way or another.

https://github.com/msgpack/msgpack-c/wiki/v2_0_cpp_adaptor
https://github.com/msgpack/msgpack-c/tree/cpp_master/include/msgpack/adaptor