you still use youtube lol, Nah seriously as @cowboysblog and @sergiomendes mentioned
it could be a number of things like browser, IPFS (dtube storage platform, which dtube maintains the servers running on it)
i'm going to assume IPFS because its the most likely the dtube IPFS API may be slow or the read/write speed could be slow on dtube servers. since IPFS is community driven (and who shares/stores the files is too) people can setup and link their own IPFS nodes to help dtube. we really need more community support for video storage i hope someone starts to promote this issue and eventually get strong community support. Hope i shed some light on this.