VW Losing Acceleration at High Speeds

Hi everyone,
This is my first post/thread here and I'm thrilled to have found this place :biggrin:.
I've recently bought an old-ass Volkswagen Gol 1.8L. This model was marketed in South America and China so don't be surprised if you've never heard of such a VW. The car was in awful condition when I got it and it's serving me as a project car. I've made some great improvements so far but I've got a long way to go. :biggrin:
Right now, the issue that I have is that the car doesn't accelerate properly at high speeds. My 0-100 kmph (0-60mph) takes 19 seconds! Though it should be something around 12 seconds. Everything is fine up until 65kmph ... the car gets sluggish thereafter and it barely gains any speed once I hit 100kmph.
https://i.ibb.co/hdPJdRs/Sudoku1.jpg
Here's my 0-100kmph time.

Here's some info:
> I've recently changed the fuel filter.
> I've checked the injector nozzles and they're clean.
> Air filter is fresh.
> Spark plugs have been recently changed.
> This car has an ECU and uses a fuel injection system but still has a distributor. I've tuned the distributor and I think it's good.
> I've checked the catalytic convertor and it's not clogged.

So with all this being said, can you guys tell me what could be causing my car to get sluggish at high speeds? Thanks a lot!

How many miles? Turbo or NA(normally aspirated)?

I would say there is a restriction, either intake, exhaust or fuel. I have heard of older vehicles having a partially clogged fuel pickup in the tank, which caused a reduction in power at high demand.