Plenty of scenarios where someone could be 5 seconds quicker. The key question is whether their lap times are beyond the capabilities of the car.
I’ve never seen anyone cheat in this version of FM (saw it from time to time in the FH games). That’s not to say it doesn’t happen but I’d be surprised if anyone is seeing it nearly every race.
On this game I’ve raced against players who cheated. There driving lines are horrible, instant acceleration like a rocket was lit. Again the zigzag black marks on the track, the car had 4 tires so why is it laying one black mark from the center of the car the whole way around the track…? More grip than me. I know it happens. Anything online can be exploited in some way. Nothing online will ever be 100% safe and untampered.
When I race the nascar oval tracks I can see the networking issues. Cars lunging forward from time to time, or instead of smoothly going around the track in the distance there movement is like skipping? Hard to describe. But yea I can spot network issues over a cheater easily.
I never questioned whether or not you can point out a cheater. From the scenario you replied with, one possible explanation besides cheating could be network problems.
You playing on those Chinese servers or something? 
must be