Sounds like a high-resistance joint.(when it's all warm and expanded, the problem goes away!)..worth cleaning all "earth" connections...including the tags on leads, any washers, metal of engine-casting etc. a smear of Vaseline (petroleum jelly) should be applied to all bright, clean, mating surfaces before reassembly (don't forget the bolt-threads and underside of the heads.
"Pos" connections should also be cleaned (a brass or Stainless-bristle brush is useful) but a smear of ordinary lithium or calcium-oxide base grease is preferable....Vaseline is conductive and a too-generous smear could provide a leakage-path to discharge the battery, pos to earth.
a lazy starter can be caused by sticking brushes, brush"tail" caught on the brush-carrier, weak , mis-placed or broken brush pressure-spring....or,not unknown, Field coils partially-shorted/earthed...Armature solder thrown from Com connections (rarer since they are usually welded/crimped nowadays)
or armature-bearings worn so it binds on the pole-shoes.
A follow-up on the cause and resolution. of your problem would help everyone's understanding....please?