This is a video showcasing the acceleration/speed performance of the second gen class 377s built after 2010 with more modermised and updated technologies, including the traction which is developed from the older variation (despite sounding very similar, they do have some slight added audiable layers though)
Electrostars on third rail are known to have some very limited power uptake capabilities and are worse in multiple sets, for example, an older Gen 8 car set will accelerate almost 20 seconds slower to 60mph than a 4 car set. In 10 car formation using older units (e.g 3 car+3 car+4 car) will also acceleration very similarly to an 8 car set.
That being said, Class 377/6 and 7s are a interesting case. At 5 cars long they are capable of great third rail Acceleration, almost on par with class 378s, but lack the same starting acceleration capabilities as class 378s, however have exceptional mid speed and higher end speed acceleration. however in 10 car formation, featuring 2X5car second-gen 377s, despite being limited further in 10 car formation, they are faster than any 8 car class 377 or 375 set, which is very odd comsidering they have more carriages in 10 car formation, but thats good none the less.
They take just around 70 seconds to hit 60mph, and have slow BUT STEADY acceleration towards 70mph regardless, and do so in decent time too. this is why i like the modernised Electrostars. A couple factors at play here, (probably) , it is entirely possible that alot of onboard electrical appliances/undercarriage electrical boxes have been updated with more efficient usage. meaning less overall power consumed for the train , and more power directed at the wheels as a consequence. Secondly , the development of the traction may make it more efficient on its own to be feasible to use more power at the wheels. and third option, could be that requirements for the newer class 377s are that they need a good standard of acceleration for metro/suburban routes. Id wager its a combo of all three factors, they are newer after all.