After a high electricity bill from her previous tenant (who no doubt
used a 3kw fan heater to compensate for a weak radiator) the landlady of
my new place - I am a lodger - is insisting that I exchange the 100w
incandescent lightbulb (says 1600 lumens on the box) originally in place
with a very much inferior 240 lumen 5w energy saving replacement. On
turning it on, nothing happens for about 3 seconds and then the room
gradually brightens over the course of about 20-30 seconds which is VERY
annoying.

Not great for me, (but not bad enough to motivate me to move just yet)
and I tried explaining that the difference in electricity - I only use
it at max between 7-7:30am and 6:30-11pm five days a week (not in at
weekends) will only be less than 3 units weekly and offered to pay the
extra, but she demurred.

What suitable low energy usage (and cheap) replacements are there that
give instant light at the flick of a switch and don't give me a headache
straining to read? It's a normal 230v bayonet fitting.