I applied strength 10 to iron golem with this command:

summon minecraft:iron_golem ~ ~ ~ {ActiveEffects:[{Id:5b,Amplifier:10b,Duration:9999}]}

However, its damage didn't increase(killed blaze with two shot, like normal).

But when I summoned iron golem first then apply strength using this:

/effect give @e minecraft:strength 9999 10

Its damage increased correctly(killed blaze at once). I tried with speed buff, it also didn't work.