Car dragged in traffic?

I drive a Kia Picanto GS, which was purchased second hand last year in 2015.
Since then in June of last year I have had the battery replaced and also had it
serviced, although it wasn’t a full service. I have never really had any issues
with the car, it did die a few times last year but since then the issue has been
resolved.

However, today I was driving and was stuck in traffic for quite a long time, an
hour altogether and halfway through progressing in traffic, so 30 mins in the
car began to produce to this burnt odd smell that I have NEVER smelt before
since driving this car. It also started dragging first gear, so when I would start
up in first gear the car would not move forward smoothly and that is never
something the car has ever done and was quite concerning. I had to press on
the acceleration a lot just to get it to move forward, usually I change gear
around the 10mph but this time even when the car had reached that the car
was dragging so second gear was dragging too.

I did thank goodness reach where I wanted to reach, and had someone look at
it. the engine was overheated and the smell was very strong. when I opened
the bonnet the heat was quite overpowering but I don’t think there was smoke.

For the timebeing he said drive it home and then have it checked, it was fine
on the way home although I’m not sure whether its my imagination but I think
the car is a lot less light and smooth than it was before, I’m just wondering
whether there is any idea anyone has as to what this could be?

I realise that without having looked at it, it would be very difficult to accurate
predict what it could be but at this stage any suggestion would fine, thank you