Edo Hiruzen is not weaker than old Hirzuen. Old Hiruzen couldn't even fight with clones against Orichimaru, as that anbu even said. Only when he was prepared to sacrifice himself is when he decided to go all or nothing.
His age doesn't affect his Edo body, his speed and stamina are no longer affected. His Edo feats alone compared to his alive feats make that pretty clear. All the jutsu he used when he fought Orichimaru, he used on a bigger scale as an Edo. Tobirama who was the fastest ninja of his time, arrived same time as Hiruzen to the battlefield.
His strength as well didn't seem to be affected either. When facing oro, Hiruzen noted that Enma felt heavier than before. However as an Edo he was easily wielding it and breaking the god tree with little effort. Edo Hiruzen is as close to prime Hiruzen as we're gonna get.
And I don't believe "prime Hiruzen" was ever stronger than Hashirama. The hype of Kabuto and Madara pretty much prove that.
It's no different to how Edo itachi > alive itachi. Alive itachi cannot use the MS to the same extent as the Edo version. Same as Edo Hiruzen, all the drawbacks which once hindered him are gone.