There is no on-purpose delay if you do them all in one action vs multiple actions with one each, though I wouldn't be surprised if the latter takes maybe a few milliseconds longer in all given that it's just a bit more work for the rule (but this is not something I'd rely on). You could also manually insert delays in between that way, though that's probably not what I'd do.
For the second question: yes, you can use metering in rules. It is only available for custom actions. Thus, instead of using Control Switches, Push Buttons > Turn switches on/off, you'd use Set Variable, Mode or File, Run Custom Action > Run Custom Action, and use the on() or off() commands directly that way (you'll be asked to choose a capability first; anything the device supports will work, though "Switch" is a likely good candidate).
Well, half of this question. I'm still fighting a device (seems random which one) that doesn't turn on/off when it's supposed to. I have all devices on text logging and don't see any spammers. Sorry for repeating myself as I'm pulling my hair out trying to fix this.
Are any of these devices Z-Wave? If so, it might be worth checking into your Z-Wave network health from Settings > Z-Wave Details. There aren't any hard rules about what's 100% "bad" based on any data there, except that "ghosts" (at least for powered devices) are generally bad news, which could be anything without "Route" information shown (unless that's just empty because there hasn't been any communication since the last restart) or an associated "Device" (or one that doesn't work, though that could be due to other problems). You can find more information on the stats here: Z-Wave Details - Hubitat Documentation.
Zigbee networks can certainly have problems, too (e.g., do you have any smart bulbs paired directly to the hub? how many repeaters and non-repeaters total?) -- but they seem to be less picky. LAN/cloud have their own concerns but most people don't have as many of those devices.
Anyway, just something else that could be worth looking into if you haven't. Metering can certainly be helpful, too, however.
My setup is 100% Z-Wave Plus. My mesh looks okay- 12 devices go direct, 5 go through 2 hops, and 2 others at 3 hops. I've had a Zen34 button that was sending out tons of battery messages, but I've excluded that now and it didn't help. Right now its like every 3 days there is one device that doesn't turn on. I have a lot of Leviton switches which have mixed reviews- now I'm messing with trying generic vs dedicated driver swaps on those. The frustrating part is it takes like 3 days before I can see any results.