MTU is always the answer (until it is not)
Small pages loaded, large downloads hung forever. Classic path-MTU trouble: somewhere a hop drops packets above a certain size and the ICMP that should say so never arrives.
ping -M do -s 1372 example.org
Walking the size down until it passes tells you the real budget. Every encapsulation layer takes its share, so tunnels on top of PPPoE end up much smaller than the 1500 everyone assumes. Clamping MSS on the router fixed TCP; for UDP I simply lowered the interface MTU.