Did you profile your program to see what takes most time and are you running in release?
I first thought to recommend PBO and GL_PIXEL_UNPACK_BUFFER but it's a sort of advanced topic (it'd require you to use OpenGL functions yourself, SFML doesn't provide it) and might help just a little and/or require you to restructure your app a bit:
http://www.songho.ca/opengl/gl_pbo.html#map1280*720*4*60 is around 200MiB/s
But this isn't
that much these days if you think about it, especially for a RAM/CPU to GPU bandwidth.
On my relatively new 'gaming' MSI laptop in the test program from the link above I can get 10-20x that on my integrated Intel and 20-30x that on my GTX 1050 (which is on PCIe v3 x16 according to HWiNFO64) depending on whether or not I use PBOs.
A single PCIe v1 lane did 250 MB/s max and that's properly ancient by today's standard. Unless there are other data transfers going on or I'm missing something this speed (~200 MB/s) is completely put to shame by everything except 100 Megabit/1 Gigabit Ethernet, HDDs and USB 2.0. It's below USB 3.0, 3.1, all relatively new SSDs no matter the protocol and form factor, Intel Optane memory, RAM disks, 10 Gigabit Ethernet, etc.