many think saitama already reach his limit after training and you know wht ? you wrong about that , saitama was a joke character ( mean a bug character )
the original author already say saitama didn't have limit plus he getting strong by doing normal thing or fight strongest being so as people always say ( can he beat Goku ?)
the answer is yes , so as you know Goku use ki and saiyan bloodline to become more powerful plus king yama already improve his biology to adapt in fight
with all this its mean Goku get help by outside and he need his bloodline , so what if Goku didn't have this ???
in dragon Ball there many things was bug so if I was not wrong , saitama getting strong by fight. Just make him fight Goku everyday ,after all every time he fight strongest being he will become 10× more strong
so from this we can all say saitama was bug and without any help become strong while Goku need king yama to improve his biology and he have Saiyan bloodline
so as this I call saitama power adapblity ,fast growth maybe more but that all for now