Failover is far too slow

VLAN failsave is slowish, but that is to be expected (the timer is 10 seconds on it lowest and like 90 seconds default) and has no influence if you take the active unit out fully. it is useful to detect no traffic on a network, so an issue somewhere else in your network not on the big-ip.

as you have noticed the failover happens, pretty much instantly i expect if you shut down the active unit.

but the traffic doesnt reach the now active unit. have you tried sniffing for the traffic to see if it actually reaches your big-ip?

as pointed out before the issue feels outside the realm of the big-ip. do some basic network trouble shooting, try to determine if the http traffic actually reaches your big-ip. if it does, then try to determine if the secondary unit can also reach your nodes?