It isn’t just high tier, anything from Tier 3 up was nice in terms of LP rewards(I think tier 3 was 50k per mission and tier 4 was 75k LP per mission, but it has been a while so I might be misremembering). I used to make about 1-1.5 million LP for 2-3h work per day - basically 1 mission for each FW agent in a faction (so 15 or so total). Not sure why another poster said they were boring, I think they must have thought FW missions were FW plexes (they are entirely a different thing).
The real problem that so many struggled to overcome was cashing out. Many people, myself included at one point, were sitting on millions of LP without an easy way to cash them out. However, the trick as I eventually learned is not to cash out immediately, particularly if the faction is at a high tier, because everyone else is doing the same. Instead, wait for the tier to drop, and the datacore offers would rise to about 1k-1.2k isk per LP. Now, that isn’t 2-3k LP I’ll grant you, but you have to remember you were earning 500k-750k LP per hour, and cashing that amount of LP out without crashing a market is very difficult. Datacore demand is so high that it would be harder (but not impossible) to crash a market. So you just wait until the cash out price is of a level you were happy with and then cash out to buy orders at that point, I think I settled for 1k per LP in the end (more than ammo for the large part and you don’t need to manufacture/buy the t1 ammo first).
edit - this is why some fw mission runners started to farm missions for opposing sides. So that no matter which side was in the ascendancy, there was another side whose tier was much lower and hence their LP was worth more. So you’d farm say Minmatar and not cash out the LP. Then when the Minmatar tier dropped you’d start cashing out your minmatar LP and start running FW missions for Amarr. Then when Amarr dropped, you’d switch again.
edit 2 - I’d say the isk per hour is broadly equivalent to running level 5’s. But, it takes a lot of patience to cash out profitably.