You should always use the latest version for your OS, as it will include the latest security fixes. The speed problem is not with Tor Browser/the tor network. Right now I am getting a 30Mbps download speed through Tor Browser - and my connection goes over a VPN first.
What sort of speeds do you get with a regular browser? Have you tried your computer on another network (friend/work etc)? Have you tried another location with a different ISP?