Hardware acceleration for Flash videos is enabled by default, but it is only supported by NVIDIA cards. So this error message will appear, if the library for NVIDIA's VDPAU backend is installed on your AMD/ATI based system. So either delete the library or ignore the messages.
Failed to open VDPAU backend libvdpau_nvidia.so: cannot open shared object file: No such file or directory
I'm not sure what might cause the slow performance (even on sites without flash content). Try to monitor the running processes while browsing some websites.