Most killing is still done at the "halt" points in the strike (like killing waves so dinklebot can open a door, killing 3 servitors, etc.) The relatively small amount of mobs you are killing in between really don't account for that much. Spending tons more time to do that clearing actually can hurt though. Say it takes 10 minutes to do a speed clear of some strike. If it takes 15 minutes to do a regular clear, the speed runner can complete 3 runs in the time it'd take you to do 2 runs. That means they would get a definite 2 blue+ items from the end reward more than you. Since you both still have to stop and do most of your killing at the "halt" points, the number of adds a cleaner would kill compared to a rusher isn't all that much higher, definitely not 2 extra blues every other run (only counting blues from the extra mobs, which would be hard to actually test because of RNG). "rushers" are still killing tons of mobs, just they pick to kill only the ones really needed to continue forwards