It should do, when I had my 2.5NA it ran very well, it would sit on the motorway at 65-70mph, needed a little time to get going but it was a good engine.
It was in good order mechanically but the injection pump eventually let it down, and it was like yours struggling to do 45mph, took some time to get it to that speed too.
I swapped the injection pump for one I got from a LZ member and that sorted it all out as at the time I didn't have the time to rebuild it I needed a quick fix.
It could also be other things, so don't go to any expense yet, is there any blow back from the dip stick tube, take the dipstick out and put your thumb over it, any pressure?
Does it blue smoke/use oil?
Is the air filter clean? Try taking the filter out completely and go for a drive (watch out for big puddles).
Is the fuel getting through well? Could be the filter is blocked.
Lots of things to check, it could be very simple, or it could need some attention.
