Found here, the claim:
...the size of the empire created problems. The sultan was expected to lead the army, setting out with it each spring from the capital. This meant that as the frontiers expanded, it took the army longer to reach the enemy, thus shortening the campaign season to the point where it was very hard to conquer new lands.
Seems like a bit of a silly problem for a militarily advanced Empire. Surely bases could be set up near the frontier from where to launch invasions. "Invasion launch-pads" are an important feature in warfare for a reason.
Was there such a strong tradition for Ottoman sultans to start their campaigns from the capital that they couldn't make a militarily sensible change? And was this whole thing even a major problem or just a minor issue beside the other ones?
Yeah there's quite a lot to dispute in this article, starting at its basic premise of Ottoman decline. It would take too long to get into but suffice it to say that the idea that the Siege of Vienna was some sort of "High Water Mark of the Ottoman Empire" after which they were destined for continuous and inevitable decline culminating in their dissolution after World War I is not really the current understanding of how Ottoman history played out.
Obviously an empire as massive as the Ottoman Empire was going to be unwieldy in the era before mass communication. But there absolutely was not an expectation that the sultans had to personally lead their campaigns from Istanbul. That simply was not how the Ottoman empire fought wars or administered territory, and of course the Ottomans were often required to have their attention on multiple fronts at the same time. If reaching Vienna in September was such a blunder in 1529 then why did they also fail to take Vienna when they reached it in July of 1683?
Local governors had significant authorities to raise troops and use them at their discretion, including for conquest.
To use a very exceptional example that nonetheless illustrates the point: when the Ottomans needed to re-establish control over Egypt they sent Muhammad Ali, an Albanian, with a few hundred troops to assume leadership of the territory. Before ultimately rebelling against the Ottomans in the 1830s Muhammad Ali raised a local army and used it in campaigns of conquest in Sudan and to re-establish Ottoman control in the Arabian peninsula.
Naval power was also a critical component of Ottoman authority, and supply lines or personal leadership from the Sultan clearly had very little to do with the career of, say, Hayreddin Pasha.
I also would take issue with huge sweeping statements like: "after 1600, the Turks lost their technological and military edge." Losing that edge didn't prevent them from attempting another massive siege of Vienna at the end of the 17th century. It didn't prevent them from fighting passably well in multi-front wars against both the Austrians and the Russians as late as the end of the 18th century. Moreover, to whom did they lose that edge? To their erstwhile rival and ally during the Great War, the "corpse" shackled to Germany, Austria-Hungary? Or to their other primary enemy, Russia, plagued with the same issues of illiteracy, lack of industrialization, crushing poverty and backwards state administration?
I would mark the real point of departure as being the 19th century and the invasion of Napoleon. The Ottomans, like all of Europe, was completely blown away by what Napoleonic France was able to put into the field. They also did not make wise grand strategic diplomatic decisions for most of the 19th and 20th century, particularly when they decided to back the central powers in World War I which was their ultimate downfall.