I want to tag on about the tension pin. Is it gummy with old oil? Is the pist bent, not allowing the pin to move? Is it the right pin - if some osmg cobbled one out of a nail it could be incorrect length or thickness? Is the hole behind it clogged with goo? And in some cases does what ever is behind there engage the pin?
I had an industrial machine with a drop in bobbin. Speed was not a problem with thread bending. I kind of think that drop in bobbin not fast theory has holes somewhere.