Engine v. sluggish when cold.

[Big Sam]

Sorry if this has been covered before! My 1990 G 2.4td has very poor performance from cold.It feels like there's no turbo boost at all until about 5-10 mins into a journey,when it performs normally.Once warmed up it's ok.
I've put a boost guage on and It's maxxing out at just over 0.5 Bar(7.5psi).
What could be wrong? Is the boost pressure ok? btw what fuel cons is normal? I get about 26mpg round town.Thanks!