And it also need bars on my opinion which extras are filled how. I did not get the point first how the boost is working. It would be nice to have some bars where it is shown how many is left of your extra powers and how is your speed status etc.
I really do not know what the speed is depending on. Sometimes I got really a slow speed when I get out of boost, but I kill several. And as it someone7070 told it is lagging and you lost your horde. Funny game, fix the bugs and it will be OK :)
There is an another bug. At objective with plane, if you kill the hippies, objective will be completed, but as mattex2 wrote you can not complete the hippy mission.
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