QCacheItem::read does a lot of small, operator based QDataStream reads.
That is be fine as long as the underlying device is buffered, which is the case in ::metaData() call.

But during the actual ::data() call, the cachefile header is re-read, this time with unbuffered device, see qnetworkdiskcache.cpp:415-418:
if (!file->open(QFile::ReadOnly | QIODevice::Unbuffered))
return 0;

if (!d->lastItem.read(file.data(), true)) {
This is causing heavy I/O and an order of mangitude slowdown of given operation on small cache entries