Starts off Slow then Play like a Pro

Ok, how does this work.  I know my playing level. I play a guy who's ranked considerably lower. Have the player with time winding down, they move pretty slow throughout the game, making mistakes (therefore I have the advantage). 

Then all of a sudden with let's say 30 or even 10 or 5 seconds left, they start playing like a pro.. no mistakes, setting moves in advance. As if I'm playing a 1500+ type player. Otherwise, they would have squashed me in an instant off the bat. 

So is this some kind of special chess engine? With advance move compatibility.  I'm genuinely curious. 

It's not odd to play slowly at first then speed up. I'll usually do that with my first game or two of the day.

It is odd that a player rated lower than you can by your estimation play very well, and do so very fast.

Although even then, lets say I'm facing a higher rated player and it's a difficult game for me. But then they blunder and now my position is easily winning. I can practically premove the rest of the game to win. I don't know what your opponent's rating was... moving instantly every move would be suspicious of someone rated around 1000. But greatly speeding up in a winning position (where you, the opponent, can't make any real threats) is normal.

There are automated programs that cheat. I don't know if you can turn them off and on during the game... I'm sure it's possible... but it wouldn't be the first thing I think of if my opponent speeds up. What would make me suspicious is a combination of two things: same speed for every move (lets say every move played after 1 second for both easy and difficult moves) and when I check it against my engine, every move is the #1 or #2 choice of my engine.

In other words not just speed, but the same speed for every move, and not just "good" moves, but an engine's moves.

I know nothing about engines but how pro are we talking about? 'Cause sometimes at ten seconds or so left people just move on instinct and 50/50 it works out like they're freaking Carlsen or something. But if they're moving lightening fast and changing their style altogether...well maybe you should report them. 

I wouldn't be too worried though. Nearly a hundred percent of the people I've played are fair(or so I should think). I bet if you play more games you'll forget about them soon enough.