Ford Fiesta 1.2L 2000 Acceleration Problems?

My Ford Fiesta 1.2L is having this strange problem.

So when i start my car up in the morning (its now cold in the UK mornings) it starts fine and all is good. When i set off, all goes well 0-30mph is no problem at all (although i do get quite a bit of white smoke out of my exhaust when first heading off but this stops after i get to speed)

But getting to 40mph takes a good 5 seconds. Not that much of a problem, But then getting to 50 takes between 8-10seconds which is quite bad and getting to 60mph takes up to 15seconds. It never used to do this and i have to stay in 4th gear all the way up to about 55mph to get it to go 60 a little quicker.

What could the problem possibly be?

I own a garage in uk… You are gonna get a lot of guesses.take it to a garage. But it does not sound expensive

White smoke - almost certainly steam which shows up more on a cold morming after a cold start with a cold exhaust system and is normal
Many cars now have a system which limits power whilst the engine warms up which obviously takes longer on a cold morming start. If this then goes back to normal after it has warmed up then there's nothing wrong with it.
Unlikely to be a fault if the engine management light is not on