it's the choke as previously mentioned, your choke isn't opening up all the way, and when the engine is "cold", it needs your foot constantly on it to open the choke alittle bit till it warms up (choke finally opens enough) and when it's completely warmed up you mentioned it's sluggish at low speeds, either it's too far opened (maybe the auto choke is worn or not reading engine temp correctly) when warmed up or not opened enough when fully warmed up.
i've never had luck with auto chokes, i've tried 3 times on 3 different cars, so everytime i see one i immediately pull it and replace with a manual mechanical cable choke.
chokes are super sensitive when cold and after it just ran across the city and then sits for 30min (partial warm up), those are the hardest for a vehicle to start, even modern cars with worn sensors.
also if it still has the original hitachi carb on there, i would say it sounds like it's working better than intended.