The Soviet Economy wasn't completely inept. They posted solid and consistent gains in terms of GDP and capital investment. In per capita terms there is similar growth. It's important to note that through much of the period the Soviet Union was a solid number two in terms of GDP after the Second World War. Even the CIA estimated 2-6% annual growth. It already had an economy larger than any rival and a population that dwarfed any developed nation with the exception of the United States.

From the 1920's and 30's when people were openly envious of the numbers the Soviets were reporting to the early 70's the Soviet Union was doing as well as anyone in the world. According to CIA Documentation in 1970 Soviet GDP was 60% of the United States. Then Soviet growth began to slow, as most of the easy gains from industrialization had been gotten and negative trends began to really emerge. Between the growth of informal economic structures that undercut the effectiveness of central planning and the central planning being simply overwhelmed by the volume of decisions that needed making many of the economic command elements that were an advantage became a liability. The Soviet Union didn't adopt computerization or mechanization as readily as its rivals, by 1970 Soviet programming was estimated to be 8-10 years behind and the US had 50 computers for each one in the Soviet Union. The Soviet government doubled down on natural resources, developing oil and diamond resources, at the expense of selling marketable consumer goods on the world stage. When there was a shortage of cash the Soviet leadership often used oil as a crutch. The apparent ability to just drill their way out of problems hid a lot of the deep structural problems for the next couple of decades.

By 1990, the Soviet Union's economy was a mere 36% of that of the United States. This wasn't because the Soviet Union, for all its manifest flaws, wasn't growing. It was that the GDP of the United States grew from $520 billion in 1960 to $5,979 billion in 1990. Japan's grew from $44 billion to $3,103 billion in the same period. The Soviets capped out at $2,100 billion in 1990. Just for reference today the United State's GDP is $18 trillion and Russia's is $1.86 trillion. It's hard to imagine the sense that Russia was reaching parity now, but then? It seemed like a given in the 50's and 60's.

What happened in the West? Well, computers made just about everything in the west more efficient. Automation replaced a lot of make-work jobs with ones that were way more productive. The US, Japan, and Germany could suddenly make way more stuff way better whereas the Soviets missed the boat. Suddenly all these other economies exploded, growing ten fold or more unexpectedly whereas the Soviets were merely meeting growth targets. After a couple of decades of this, it simply wasn't a contest anymore.

In short, the Soviet Union was holding its own in every way until about 1970, but the wheels didn't come off the Soviet Union so much as everyone else strapped on rocket boosters. Rocket boosters that were simply too dangerous politically for the Soviet Union to throw its full weight behind.