Canadian international Peschisolido masterminded the Rams' raid as Derby snatched all three points with a goal early in each half.
The first, after eight minutes, was an unfortunate own goal by luckless Pilgrims centre-back Graham Coughlan, who turned Peschisolido's cross into his own net; the second, in the 62nd minute, was an exquisite finish from the Canadian international.
"The scoreline was a fair scoreline," Williamson admitted. "Credit to Derby. The two lads up front was probably the best pairing I've seen in this division so far.
"Their linkage was great and their movement was very, very good and they worked very hard for the team.
"I'm as disappointed as the next man who supports Argyle.
"We had good chances and, if we had taken them, it might have put a different reflection on the game, but we've been saying this for a few weeks now. I seem to be repeating myself on a weekly basis."
Derby boss George Burley confessed that he was enjoying his side's current run, which included a derby victory over Nottingham Forest last week.
He said: "Our last two performances – the Forest game and today – have been outstanding. I can't ask any more of the players, and I have enjoyed watching the team at times and that is good for the manager.
"Some of our stuff in the second half would have caused most teams problems."
Derby were lucky to remain ahead after Coughlan's own goal when Argyle forward Micky Evans spurned a free header six yards from goal after being teed up by David Norris.
However, after Peschisolido's second, Argyle were forced to open up and Derby had several chances to increase their winning margin, with Argyle goalkeeper Romain Larrieu needing to be in outstanding form to deny Rasiak and substitute March Reich.