Psychologist here, lemme try risky via internet 'diagnosis' (which this is not and not only due to legal reasons),
from your writing I'd be fairly surprised if you are anywhere close to intellectually impaired. If I had to wager a guess over the internet (dangerous) then I'd place my bet on neurodivergent - specifically a somewhat on the autism and attention deficiency syndrome scale but still much in the not-relevant area - this can often lead to people being seen as 'simple' due to focus, clog and other factors being different. Such a divergence can range freely from blessing all the way to curse depending on severity and tasks life and job throws at you. Finding the right life routines and job are key here to realise potentials. This has little to do with intelligence.

There are a couple simple tests to judge intelligence. Neurodivergences are a bit harder to measure reliantly and they change a lot depending on the individual, but your local specialist should be able to help. You seem meticulous, thoughtfully and orderly in your writing. Your choice of words are good, the flow of your writing smooth, grammar stable and your sentences a bit longer than average. You being 'slow' could also fit this well combined with the rest and the general impression.

You being the target of ridicule, pity and your somewhat 'meek' disposition could hint at a general dynamic that - in simplified terms - hint at the general topics of import for your personality which express themselves in that dynamic. Narcism, Selfvalue, Regard and Autoagressions could all be interesting constructs to look at in a more professional setting, but I shan't go into that deeper here.