It shouldn't be taken for granted that the forces stationed in Manchuria were materially much worse off than those fighting on active fronts in China and other places in the Pacific.

The best units and men, combat and non-combat, were pulled from duty in Manchuria as manpower shortages and crippling losses meant those reserves were needed elsewhere. What was left was a mish mosh of reservists, draftees, local militias formed from pioneers and settlers, and reorganized units that had often suffered catastrophic casualties and were no longer combat effective.

Even a masterful commander would be hard pressed to organize this into an effective fighting force. Never discount organization in drawing out war potential. A mob of 1,000 can be put down by a well drilled force of 100.

On top of this, the forces in Manchuria were not well supplied at all.

They were heavily lacking in both armored support and artillery pieces. Their firearms and ammunition were surplus or ancient, and when they did have modern equipment, they desperately lacked ammunition, spare parts and other miscellaneous materials required for upkeep.

Food and medical supplies were not in the best supply either.

Finally, the forces stationed in Manchuria were constantly facing insurgencies and partisan attacks. It should be remembered that while Manchuria/Manchukuo was nominally an indispensable part of the Empire, it was relatively newly conquered and settlers were often in combat with bandits and insurgents alike, both of which were part of the constant indigenous resistance to the Japanese occupation.

All in all, even in the best of cases, the Japanese forces in Manchuria were standing on very unstable ground and were not in top shape to fight a pitched defensive battle against the Soviet invasion.

We haven’t even touched on the Soviet strategic advantages and broad tactical successes in actually conducting the invasion. For example, they caught the Japanese forces by surprise and completed a half envelopment of much of the Japanese positions while penetrating deep into Japanese territory.

All in all, the Manchurian invasion was not the best showing the Japanese forces could have had if they were focusing their efforts there like they were in China or against the American push in the Pacific. But strategically, this would likely never have happened simply because it couldn’t be done. What the Japanese had in Manchuria was what they could afford to keep there, already fighting on two major divergent fronts. There was little they could have done differently even with hindsight.

So it feels like there are a few parts to this question and hopefully I can speak to a few of the parts of it, if I answer something you weren't asking please feel free to clarify your question.

Basically it feels like you are asking: "The USSR made fast progress when it attacked in August of 1945, why did it take the US 3 years to advance to the shores of Japan"

First lets look at why it took the US so long to get to the shores of Japan itself.

It was essentially an active strategic choice. Early versions of War Plan Orange, and Japanese plans to win essentially relied on the Pacific Fleet rushing to the Western Pacific to save US garrisons(most likely the Philippines), and once there fight the IJN and begin a blockade of the Home Islands. But there are problems with that plan, namely that the fleet will have no good developed forward base, nowhere to refuel and rearm in safety, where damage control and light repair can be done and supplies stockpiled. Thus any rush to fight would likely end just as the Russians experienced at Tsushima.

So the alternative is a deliberate strategy of island hoping, to take a position you feel will help turn it into your forward base, and then grab another one, so your forces are never too far from safe harbor. With the rise of aviation this became doubly important, as islands of sufficient size could serve as patrol and eventually bomber bases to hit the Home Islands.

That however takes time, to build enough support ships, to build train the men in all the tasks, to stockpile the supplies. And it was a major learning curve. The first attack, the invasion of Guadalcanal in August 1942 was run on a shoestring with numerical inferiority in virtually every category, from ships to planes, to support craft, but determination, some luck, and underestimation by the IJN/A let the Marines dig in sufficiently till the Navy could secure control of the seas to bring reinforcements and more supplies.

From then on though instead of one single push the US took a 2 prong approach, in order to confuse the enemy and never give him room to rest. From Australia and the Solomons forces under General MacArthur pushed along New Guinea and towards the Philippines, while the Pacific Fleet under Nimitz pushed from the Marshalls into the Marians(where islands such as Saipan and Tinian would eventually serve as the base for the B-29's to hit Japan including the Atomic Bomb missions).

These were massive undertakings, the invasion of Saipan, launched on the other side of the globe one week after Normandy involved 300k men and 600 ships. Including 15 carriers and almost a dozen old and new battleships. But in the end it still came down to men and guts ashore, Saipan's garrison despite being outnumbered almost 3:1 held out for almost a month. Across the Pacific on the islands you would see this, casualties would be massively skewed towards the Japanese, because there was no retreat, every cave and bunker might be cleared 2,3, 4 times before secure, and until almost everyone was dead, of the 30k defenders only about 1k were taken prisoner.

However the schedule was mostly met, both prongs advanced at a rate that let the supply end keep up, and not get too far stretched out, while keeping a strong covering fleet with both that was able to brush aside Japanese counterattacks, though not without some drama and danger. With about 3 major landings per year before the storms got too bad it allowed by late 1945 for US forces to operate off Japan with the same supplies, quality, and presence as if they had been off the coast of California, a distance greater than that from London to Moscow by a factor of 4, Hawaii itself to Tokyo is still twice the distance from London to Moscow.

Just to give you an idea of how the distance and scale of the Pacific served to force choices on base locations, and thus travel time and where to seize islands.

We can then contrast that with the situation faced by the USSR in August 1945. They had a relatively wide open area, that if remote, at least allowed for them to decide when to attack, and do so only when confident of their supply situation, and to make good use of maneuver and mechanized forces to avoid enemy strong points in textbook modern maneuver warfare.

We should also consider the type of enemy they faced, the IJA was not built for modern combined arms fighting, it was predicated on rapid advances and quick decisive flanking attacks by mostly light infantry. An entire division might have less artillery than a US Regiment. On Coral and rocky islands with a chance to dig in to fixed positions and knowledge that there was no retreat these issues could be minimized. But in open terrain against a thoroughly modern foe it was not a good mix.

It should also be noted that while huge portions of the IJA remained in China and Manchuria come 1945, many good divisions and formations had been withdrawn, Saipan for example received most of a divisions worth of troops from China(however US Subs attacked the convoy and sank 5 of 7 ships carrying what heavy equipment and additional artillery they did have).

TL;DR The US fought a war against fixed hardened points across half the face of the earth, while doing so at a pace meant not overextend themselves and lead to more casualties. The USSR was able to wait to attack a theater that had become almost a backwater, and do so while able to use their newly refined modern tactics against a foe that couldn't match them.

Good Sources I might recommend are Ian W. Toll's new 3 part series on the Pacific War, along with Richard Frank's Downfall, a very strong work on the end of the war and the events of August 1945.