Even low-speed operation can be nearly-as or as-efficient with paired injectors, when firing twice per-cycle, known as double-shot or quasi-sequential. This mode fires half the fuel on each injection. Timing is as-accurate as full sequential, and timed the same as sequential (peak efficiency timing).
Sequential fueling injects during an open valve. This is (mostly) incorrect, and there is an assumption or rumor on the internet that it fuels this way. The vast majority of systems, and especially newest OEM systems, purposely inject on closed
valves for improved vaporization and fuel distribution. The strongest effect is fuel dwell. At higher speeds and loads they may inject on an open valve, but only as a result of shorter time and greater fuel requirement, not intentionally. Speeduino is easy to tune for best timing using data feedback.
There is only one specific benefit to full-sequential, which is individual cylinder fuel trim (ICFT) that sequential timing allows. It isn't the sequential action that is the big benefit (as most assume), but rather the access to the individual injector flow trimming or calibration it provides, allowing perfectly-matched fuel flow to the airflow of each cylinder. This is the benefit to sequential, and is most useful for emissions and somewhat for economy applications at lower speeds and loads. These are the reasons the OEMs use sequential, and has little or nothing to do with peak power.
In a similar vein, Individual Cylinder Ignition Trim (ICIT) accessed by sequential can be even more beneficial than ICFT in some applications, to permit a multi-cylinder engine to run stronger than the weakest cylinder allows. Weakness can be in many forms, from production design or tolerances affecting induction, exhaust, effective compression, heat, propensity for detonation, etc. Speeduino does not yet support ICIT, but it is related to sequential in the same way as ICFT.
One other attribute that is sometimes raised, is the response of full-sequential, as it "updates" the correct fuel to each cylinder every cycle (2 rev's). While that's true, the comparison was with bank or simultaneous batch firing. Paired injectors and double-shot actually update the fuel by 50% on each half-cycle (1 rev), and so that old reason is no longer valid. Modern OEM systems now use the double-shot method that Speeduino has been capable of for years, but on sequential, to trim for best benefit on emissions and economy regulations.
I hope that too-short overview helps to sort modes and effects of each. If you have questions around this topic, please be specific with them, so short and specific responses can be offered.