Blockwars isn't popular because it's extremely boring to play with it taking 5 seconds to
break blocks, and you can't add more blocks mid-game unless you're Miner.
As for "5 seconds to kill", let's take some comparative examples. Most people in Blockwars have leather armor; I think the most-armored kit is like... 9 armor points? And I'm pretty sure the strongest weapon is a stone sword (except the tnt, which is stronger in 1.9+ simply due to the armor nerf). Before getting into it, some ground rules:
- Time to kill is recorded as the absolute minimum amount of time that a player can take in order to deal fatal damage to a full-health enemy with a given weapon.
- The timer for "time to kill" starts upon the first hit landing.
- It is assumed that no regen will occur; it would only change optimal "time to kill" by .5 seconds, in any case.
- "All crits" will only be considered for 1.9+ because you lose time in 1.8 by trying to do all crits over simply jitterclicking.
So, to kill a person in full leather w/ chain chestplate (9 Armor):
1.8: Wood Sword - 3.5s to kill; Wood w/ 1 Crit - 3s; Stone Sword - 3s; Stone w/ 1 Crit - 2.5s.
1.9+: Wood Sword - 3.75s; Wood w/ 1 Crit - 3.6s; Wood All Crits - 2.4s; Stone Sword - 3.25s; Stone w/ 1 Crit - 2.5s; Stone All Crits - 1.75s.
To kill someone in full leather (7 Armor):
1.8: Wood Sword - 3s; Wood w/ 1 Crit - 3s; Stone Sword - 2.5s; Stone w/ 1 Crit - 2.5s.
1.9+: Wood Sword - 3.75s; Wood w/ 1 Crit - 3.1s; Wood All Crits - 2s; Stone Sword - 2.5s; Stone w/ 1 Crit - 2.4s; Stone All Crits - 1.75s.
Of course, these being "optimal" situations, most of the time battles will take a few seconds longer. But demonstrably, the amount of time to kill between the versions are within a second of each other, with 1.9+ becoming faster as you consider crit combos that are more viable in 1.9+.
What you're probably referring to with your "5 seconds to kill someone" statement is the fact that 1.9+ punishes bad aim by making you have to wait to strike again. A little less-than-optimal aiming in 1.8 is usually pretty meaningless to the fight as a whole; 1.9+ puts much more emphasis on having consistent aim and timing, because one miss usually adds about another half-second to your time-to-kill timer, and gives an opening to the enemy to combo you.
Of course, if it isn't your bad aim, it could be that you or the enemy are lagging really bad, which is why Cubecraft is a pretty awful place to get a good look at 1.9+ combat, honestly -- it has awful server performance (in my personal experience at least). It would also be very beneficial if Mojang added more features to aid in pvp consistency such as a vanilla anti-cheat and lag compensation. But I digress.