So-called 'optoisolated' relay boards are almost invariably cr@p with the optocoupler implementation offering little or no advantage over a simple transistor relay driver, and having the disadvantage of requiring a significantly higher driving current from the logic connected to its input(s)
You can recognize the cr@p by the presence of tracks running under the optocouplers compromising their isolation rating by reducing the creepage and clearance distances - any designer who makes this noob mistake cant be trusted to use safe clearances under the relays either, so unless you've personally checked the clearances the board is a potential deathtrap if used to switch mains voltages even if the relays are rated for mains AC switching (and not fake).
Another sign of cr@apness is if there's any ground or Vcc connection on the board that crosses the supposed isolation gap. Again this defeats the whole purpose of optisolation.
In very high EMI environments there may be a need for an optoisolated relay board, but ideally it would bring out both sides of each opto's LED individually (with or without a series resistor) so each channel can be hooked up to the MCU board using twisted pair for noise immunity (also allowing a choice of high side or low side drive), and should probably have an anti-parallel protection diode across each opto's LED. As described above, creepage and clearance distances under the optos should be the maximum permitted by the footprint, preferably with the board slotted individually under each opto, (a tight layout with a long slot under all the optos typically results in a mechanically weak board), indicator LEDs (+ their series resistors) should be in parallel to the relay coils so they can indicate the true relay state and diagnose failed drivers, coil power to the relays should be on a separate connector to the opto inputs, clearances on the contact side of the relays should be at least 4mm between contacts and between channels, and there should be provision to fit snubbers (either series RC, TVS diode or varistor) across each contact pair. Furthermore the PCB layout should be in the documentation so any engineer reviewing the system design can check creepage and clearance distances.
The Elegoo board fails on many of the above and I certainly wouldn't use it for switching mains until I've seen the PCB layout and checked clearances. The documentation shows there's an isolation slot between the COM and coil pins of each relay, so *IF* *PRESENT* thee's *some* hope it may be safe for switching mains. The presence of tracks under the optocouplers and both input side Vcc and 'JD_Vcc' (coil power) on the same connector irredeemably compromises the optoisolation which therefor cant be trusted for more than a low voltage DC offset between the logic and coil power sources. It would be adequate to break ground loops.
Now on to its actual performance. If you consult the full Sharp PC817C optocoupler data sheet, you'll find its min. CTRR is nominally 200, at 5mA If. However Fig. 4 'Current Transfer Ratio vs. Forward Current' shows a significant drop-off at low currents, and at 1mA If, I estimate you can only count on a min CTRR of 125%. The SRD05SLC relay has a nominal coil current of 71.5mA. its min. pull-in voltage is 75% of nominal, so you cant tolerate more than 1V drop (assuming the supply is 5V -5%) across the SS8050 transistor, which has a min. hFE of 85.
Al any opto input current below 1mA you are quite literally off the charts and operation isn't guaranteed. However I *think* it should operate down to around 0.7mA when new. N.B. there is no allowance for opto LED aging, so I wouldn't be happy with under 1.5mA for any serious application. Assuming a low Vf (1.7V nom.) GaAS red indicator LED it *MAY* operate from as low as 3.3V in, but don't bet on it as if an ordinary nom. 2.1V Vf LED is used I believe it will need at least 3.6V! For long-term reliability, IMHO it needs at least 4.5V drive.
Then as 'golden_labels' has alluded to, you have the voltage drop in the ESP32 output. This *may* be as high as 10% of its Vcc, though IMHO this is unlikely sinking less than 1mA. The parameter you want is VOL, though tracking it down in the Espressif documentation is difficult, and you need to know exactly which ESP32 chip is used in your module.
TLDR this relay board isn't suitable for 3.3V logic without level conversion or modification, and may not be 'fit for purpose' anyway!
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