Originally the Clan tech base was intended to take a bit longer, evidently: HOK says that improved tech was perfected as the Clan tech later during the Golden Century, around 2850, ie 30 years after the Operation KLONDIKE.
But for whatever reason, the timeline was compressed to about ten years with Interstellar Operations (and perhaps based on some 'Mechs introduction dates). This is highly unfortunate as it is slightly unrealistic, and it reduces the time there would be to play with weird Star League-Improved Star League-prototype Clan tech mix the early Clans had.
The Clans did have an "advantage" in having less technological base and presumably more advanced tech base in general before moving on the pure Clan tech. The former is pretty simple: Fewer factories to upgrade, and new ones would be built straight to Clan-spec. The latter makes the move easier in general.
The Inner Sphere had more weapons than the Star League circa 3067, but overall their tech level may not have been that high. The ER lasers and other variations are largely based on Clan-tech reverse engineering and are logical extensions to Star League tech. As it is, upgrading Inner Sphere factories is a difficult task, you need to take them offline to make changes, but there is constant need for equipment (the early Clans had extensive Star League stocks already so they could easily take factories offline probably). There's also that many factories are likely in worlds with limited tech base, meaning upgraded tooling needs to be made elsewhere and then shipped, adding to cost and time.
The Jihad's destruction and the subsequent disarmament among the Inner Sphere factions likely slowed down adoption of Clan tech, both due to destruction caused, and due to various prototype technologies that are easier to manufacture having had matured and spread around, so presumably most opted for those rather than delay and make Clan tech upgrades. Better to have something now, than have something even better later but be without anything for now.
The Fulcrum X was likely a test by the Federated Suns to see if they could move to Clan tech. But given the evidence, it didn't really pan out, they only really cracked the mass manufacturing circa 3120s, as did other houses.