forlistpb , most important thing, Do Not upgrade any firmware. You would 99.9% most likely brick your player instead.
What do you mean by the player working very slowly? Does it take a long time to load the directory?
It is quite common for 4GB and above players to act this way. As the processor is not fast enough to handle a large capacity such as 4GB.
Anyway, i'll do suggest a format on your player using MP3 Utilities Disk Tools, just to verify that the memory is not hacked. The guide is here
http://www.mympxplayer.org/how-to-re...ck-vt2400.html