Comment by wtarreau

If you transfer large objects, H2 on the backend will increase transfer costs (due to framing). If you deal with many moderate or small objects however, H2 can improve the CPU usage for both the LB and the backend server because they will have less expensive parsing and will be able to merge multiple messages in a single packet over the wire, thus reducing the number of syscalls. Normally it's just a matter of enabling H2 on both and you can run some tests. Be careful not to mix too many clients over a backend connection if you don't want slow client to limit the other ones' xfer speed or even cause head of line blocking, though! By typically supporting ~10 streams per backend connection does improve things quite a bit over H1 for regular sites.