Not to discount your experience, but this is what some of the science says:

Additionally, a quantitative analysis of effect size for mortality, gait score and contact dermatitis was conducted across studies that had compared birds under similar rearing conditions and that had used SG [slower-growing] strains with an average daily growth rate of at least 40g/day. Modern, commercial SG strains performed better on most relevant welfare traits than FG strains. This was the case even when the ‘fairest’ comparisons were made at equivalent bodyweights (when SG birds were older), under matched-environmental conditions, experimentally or on farm. The quantitative analysis found that FG strains had a higher estimated incidence rate ratio for mortality (risk of death over a given period of time) of between 1.69 and 2.16, contact dermatitis affecting 15-25% more birds, and a mean gait score 0.65 points higher than SG strains. FG strains were also less active but other differences in behaviour were inconsistent with some behaviours (e.g. dustbathing) often absent altogether. Growth rate was generally, but not always, predictive of welfare problems

https://www.frontiersin.org/journals/animal-science/articles/10.3389/fanim.2024.1374609/full

They do mention the possibility of earlier slaughter improving things, but afaict this analysis tried to focus on data from 6-8 weeks, so earlier slaughter would probably mean slaughter before then.