I was thinking about how most beings were stronger in earlier times than in say, the Third Age.
Did this apply to orcs as well? Did the power Morgoth put into the orcs in the First Age make them stronger than the orcs under Sauron/Sauruman?
Physically they were almost definitely not any different at all. They were elves after all and had the same immortality of all their kind and much of the same resistance to wounds and all the rest. Even in the first age there were differences between Uruks and Uruk-hai - or Goblins and Hob-Goblins as they were sometimes called by Tolkien. From materials in The Silmarillion, The History of Middle earth and Morgoth's Ring it is likely that Saruman did not invent but rediscover Melkor's technique for the creation of Uruk-Hai. Interestingly the term 'Goblin' seems almost exclusively to be later applied to the particularly weak and petty groups like those at 'Goblin Gate'.
However, the Orcs as a combined class clearly 'did best' when they were being led by either Melkor himself or one of his appointed servants. Without strong leadership it seems they tended just to mill around and dissolve into internecine quarrels and back-biting. It was actually one way the Wise generally and The White Council in particular could judge the strength of the enemy - by how organized, vicious and successful the current 'Orc problem' was proving. Elrond mentions this several times in LoTR alone. Moreover, this behaviour is seen over and over during the Silmarillion, the various stories in Unfinished Tales and throughout the Appendices.
Examined more closely; obviously once his spider Ungoliante had destroyed The Trees, Melkor himself stolen the Silmarils and at first together finally made their run for middle-earth the Orcs became a terribly dangerous enemy and most importantly started to breed in numbers again. Beforehand, we are told in the Silmarillion that during Melkor's chaining they were little more than a lingering pest to Elwe's people. As the fallen Aratar regained strength once out of Mandos the far distant Orcs also began to regroup among the truly ancient ruins of Udun and Angband. Melian as was her tendency recognized this growing danger and counselled her lover Elwe and their Sindar of Doriath to buy a large quantity of arms and armour from the Dwarves of the Blue Mountains. This culminated in Melkor's return and the First Battle - The Battle under The Stars.
Surprizingly little is told directly of the actual end of the First age - supposedly because the chroniclers of the Silmarillion were not directly involved - the War of Wrath almost entirely prosecuted by the Host of Valinor alone. Nonetheless it is said that along with the destruction of the majority of Dragons and Balrogs almost the entirety of the Orcish race was also exterminated. Perhaps if the geology of Beleriand had not been so compromised by the downing of Ancalagon by Earendil and the divine power from so large a clash of Maia and Balrogs it might have been possible to totally reclaim all that had been lost to the Eldar over the previous five hundred years. However, they were tired and had enough of the endless struggle and sorrow and most of the survivors of Feanor's rebellion went home to Eldamar.
Nonetheless, in Of The Rings of Power and the Third Age along with parts of the Appendices and Unfinished Tales in particular we are shown how for a good half of the Second-Age most of northwestern middle-earth was left in fair shape. The remaining Noldor clearly able to prosper without the constant menace of Orc attacks, founding first Lindon under Fingon's child Gil-Galad in the bare remnants of Beleriand before the Blue Mountains and then Galadriel's colony of Eregion outside Moria's East Gate. Finally after Celebrimbor's 'bloodless coup' she was still able to evacuate enough of her loyal followers and build them Laurelindorenan itself. All this without the interference one would expect in later times from the evil denizens of the Misty Mountains so close at hand.
Both Unfinished Tales and Of the Rings of Power and The Third Age shows how inevitably, as Sauron began to regather himself the Orcs also began to wax in strength - culminating first in the War of Elves and Sauron, then the tandem defeats of Sauron initially by Tar Ciryatan and over a thousand years later by the massive forces of Ar-Pharazon. Once Sauron was hauled off to Numenor in chains, the orcs left behind began to wane a little... Until the Akalabeth and the pendulum swung once more. Nonetheless this slight respite did give The Exiles just enough of a breathing space to gain a foothold back in middle earth again and Anarion be able to hold off Sauron long enough for Isildur and Elendil to gain Gil-Galad's support and form the Last Alliance. This ten-year conflict mentioned in almost all of Tolkien's published works to greater or lesser extent seems like it burned through most of the remaining Orcs of the Second Age - and not a few Dunedain and Noldor/Sindar elves alongside! Once again, with Sauron defeated the few surviving Uruks and Uruk-Hai retreated to the shadows under various mountain ranges and left the world alone for another good fifteen hundred years.
From the Appendices we can see how Sauron's next slow but inexorable rise to strength, ending in the War or the Ring was again also once more attended by the increasingly focussed Orc bands and their raids. Huge inroads were made for good here and there - after the War of Dwarves and Orcs almost every Uruk in the Misty Mountains was destroyed and a century of nearly total peace followed. However pockets remained around certain cores of Melkorian remnants - such as the Balrog in Moria, a while before this within the Witch Realm of Angmar under the Black Rider Er-Murazor while it lasted and of course around the Necromancer/Sauron at Dol Guldor in Southern Mirkwood... And latterly the rogue Istar Saruman at the southern tip of the misty mountains.
So. In final summary - No. I do not think they were at all stronger in times past. They just had a stronger Lord to focus their malice around and concentrate their efforts for evil.
"Most beings were stronger in earlier times"
I am not sure this premise holds water, and would amend it by saying "Most good things have faded since earlier times." Consider:
Tolkien was much influenced by George MacDonald stories The Princess and the Goblin and its sequel The Princess and Curdie, and the end of the latter tale has a very familiar The World Used To Be Great But These Days Everything's Going To Hell ring to it familiar to Tolkien's readers.