1996 Ford Fiesta acceleration very slow. engine won't climb above 4000 rpm

I have just bought a ford fiesta 1996, 5 dr, 1.3 60ps EFI, Ghia.

The Car around town is slow, but fine. Yet if you want to do more than 30 mph there's a problem.

In all gears, the car won't climb above 3500/4000 rpm. And thats a struggle.

There's no Power from the engine. It Takes miles to get up to 70pmh.

The Gearbox is fine. The clutch is fine (CHECKED).

HT leads are producing spark.

Could it be as simple as spark plug issue? The engine has done 102,000 miles and been fully serviced up to 90,000.

I've tested comprehsion, (as best as i can) with the engine running, removing the ht leads. The pitch of the engine changes on cylinders 2-4. Yet the sound change on cylinder 1 is very miniscule.

Advice would be appreciated. Thanks

G