I have a cove stiffie that used to weigh about a metric tone. I probably rode 20000 miles on it using it for 50 mile day rides and I commuted on it 5 days a week for nearly 10 years.
It had marzocchi shiver sc's, hope big un hubs with atomic labs trail pimp rims and race face diabolus cranks some of which were transferred from my dh bike.
I changed the wheels in march to some hope pro2 evos with mavic 819 rims and was amazed how much better the bike rode.I think i worked out that the new front wheel weighed the same as just the old front rim.
I then changed the tired old shiver sc forks which saved nearly 1kg. I couldn't believe how much better the front end felt for being that bit lighter.
Lastly i have just converted to 1x9. By removing the old diabolus cranks, xt front mech and shifter and replacing with some race face deus cranks, a single ring and a chain guide it has dropped about another 1kg from the bike.
None of this was ever done on purpose to save weight but its what has seemed to make the most difference to the bike.