It's pretty hard to have more than a little hidden tech. That's because big government and big corporate labs are, with just a handful of exceptions, terrible places to get innovation. Plus, secrecy means you lose open competition with other scientists, trust me, a MAJOR slowdown.
... on the other hand, it's pretty easy to slow tech.
o Killing freedoms in lots of people is one way.
o Unchecked monarchy or dictatorship also means byebye freedoms - all the vast lands conquered by the Roman Republic went from inventive to stodgy quickly by passing from republic to unchecked monarch.
o High corruption cuts down on advances, and each succesful society has eternally advancing corruption.
o Republics generally only last 6-8 centuries before failing from corruption.
o Most effective is if you can kill USE of the scientific method worldwide, which happened a century after the Republic's death to Empire - after Emperor Qin burned ALL the books in China and a little before most of the books in India were rewritten to make sure we'd never find out how Indian rebublics and scientific method there died as well. It COULD all repeat in 4-6 centuries from now , or you could kill the Renaissance in which it finally came back after a millenium and a half.