Here's a little helpful tip for people who have unlocked Langley the Conductor and Mycall. Have Mindy mimic Langley while Mindy is to the RIGHT of Langley, and Langley is in the column to the right of Mycall. Have your DPSer in the column to the left of and adjacent to Mycall. Any ability affecting Mindy but not the others will be buffed by the mimicked "Flip the Switch" and passed on to Langley, who then buffs it again with their original "Flip the Switch" ability and gets passed onto Mycall, which then, again, is buffed by Mycall's "Germination" ability, which then finally reaches your DPSer.

Example: Pete with level 8 gear will cause his "Bad Joke" ability to buff about 26k%, but if it only affects Mindy, then Mindy buffs that to 78,000%, Langley buffs that to 117,000%, Mycall buffs that to 175,500%, which your DPSer finally receives. That's about 26,000% versus 175,500%. That's 6.75x the effect.

I have Langley in my formation the entire run. Rex is only good for spawn speed, which is somewhat useless once you have a high enough level of "Lingering Buffs". I pop about 11 speed buffs at the start of every run so that I have good speed throughout the run. Sacrificing such an insanely powerful crusader for a teensy bit more speed amounting to maybe...3 minutes...? less time per run seems wasteful. I'll gladly lose those few minutes for the sake of pushing WAAAAAAAYYYYY farther thanks to Langley. They let me push another 30 areas further with ONE legendary item UNLEVELED. That means they buffed my damage more than Dr Evil (the crusader I took out)with all three legendaries at level 3. I use Wrena as a DPSer, so that's two stacks of x2000% DPS buff I'm losing and still pushing way further despite that.

It really doesn't matter what minimal amount of speed you lose. Langley is worth keeping in at all times. Besides, spawn speed above 300% is kinda pointless. I think it takes...400% to insta-kill before they appear on screen? But at 200%, you're already killing them in about 3-5 frames after they appear. My spawn speed tends to hover around 200-300% without any spawn speed buffers in the formation.

there is a challenge store you know.

My spawn speed comes from lingering buffs. Depending on what I'm doing, I use a different amount of speed cards.

Chest farming runs: 1 rare, 1 uncommon, 2 commons. Don't need much since sprint runs only take around 50 minutes, plus having a spawn speed formation anyway.

Overnight runs: 2 rares, 3 uncommons, 6 commons, just to keep the speed up while sprinting, and a little while longer so the run is more likely to be at end-of-run by the time I wake up.

Idol runs: 4 rares, 8 uncommons, 16 commons. That tends to keep those buffs active ('cept for the rare one) for the entire time it takes me to complete an idol run. 2:3:6 is the ratio to keep all the effects at an equal length, x2 to make them last longer, then +2 uncommons and +4 commons to get that extra 90 minutes at the end of a run where the buffs would have worn off and my formation has switched to DPS.

All that in mind, Lingering gives me more than enough spawn speed for the entire run.